Methods and apparatus for optically detecting magnetic resonance

ABSTRACT

A magnetometer containing a crystal sensor with solid-state defects senses the magnitude and direction of a magnetic field. The solid-state defects in the crystal sensor absorb microwave and optical energy to transition between several energy states while emitting light intensity indicative of their spin states. The magnetic field alters the spin-state transitions of the solid-state defects by amounts depending on the solid-state defects&#39; orientations with respect to the magnetic field. The optical read out, reporting the spin state of an ensemble of solid-state defects from one particular orientation class, can be used to lock microwave signals to the resonances associated with the spin-state transitions. The frequencies of the locked microwave signals can be used to reconstruct the magnetic field vector.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the priority benefit of U.S. Application No.62/418,888, which was filed on Nov. 8, 2016, and is incorporated hereinby reference in its entirety.

GOVERNMENT SUPPORT

This invention was made with Government support under Contract No.FA8721-05-C-0002 awarded by the U.S. Air Force. The Government hascertain rights in the invention.

BACKGROUND

The nitrogen-vacancy (NV) center in diamond has a number of exceptionalproperties, including a long room-temperature spin coherence time (˜1ms), optical accessibility for spin initialization and detection, andembedding in a solid-state spin environment that can be engineered for awide variety of applications. One application for which the NV centerhas shown particular promise is quantum sensing including magnetometry.A standard method of sensing magnetic fields with NV centers uses anoptically detected magnetic resonance (ODMR) spectrum to determine theNV ground state transition frequencies which experience Zeeman splittingas a function of the applied magnetic field and are scaled only byfundamental constants. However, current NV magnetometers that employthis method rely on measurements of NV fluorescence intensity and aretherefore susceptible to noise in the optical and microwave excitationsources used to perform ODMR measurements.

An earlier approach to ODMR-based NV magnetometry measured the fullspectra and performed fits to extract the NV resonance frequencies. Itinvolved spending a large fraction of the measurement time monitoringnon-information-containing off resonance signal and was subsequentlyprohibitively slow.

Another earlier method used lock-in techniques to continuously monitor asingle resonance on the approximately linear derivative of the curve,from which small resonance frequency shifts were detected by applyingpre-calibrated scale factors. However, this second approach was stilllimited to the approximately linear regime of the lock-in signalresulting in a dynamic range of about 10 μT. Furthermore, this methodwas dependent on phenomenological variables instead of a true frequencyshift. In particular, the scale factor was influenced by the NVresonance linewidth and contrast, both of which were affected by opticalpump power, microwave power, and detection efficiency. These variablesare different for each device and drift over time, consequentlyrequiring periodic calibration.

A third earlier method employed a closed-loop feedback lock-in techniqueon a single resonance of a single NV center; however such an approach isnot inherently suited to magnetic sensing, suffering from severesystematic errors due to, for example, time-varying temperature andmagnetic field when applied to magnetic sensing. Furthermore, single-NVmeasurements are incompatible with vector magnetometry. In particular,none of the previous methods performed measurements on multiple NVresonances in an NV ensemble simultaneously; thus none were able toextract full magnetic field vector information in real-time, decoupledfrom the effects of temperature.

SUMMARY

Embodiments of the present invention include methods and apparatus formeasuring magnetic field vectors. An example vector magnetometercomprises a solid-state host comprising an ensemble of color centers,oriented along several axes (e.g., the symmetry axes corresponding tothe orientation classes of color centers) with respect to the crystalaxis of the solid-state host, a microwave signal generator inelectromagnetic communication with the solid-state host, a photodetectorin optical communication with the solid-state host, circuitry(including, e.g., a lock-in amplifier) operably coupled to the microwavesignal generator and the photodetector, and a processor operably coupledto the lock-in amplifier.

In operation, the ensemble of color centers of the vector magnetometerare driven with a first microwave signal from the microwave signalgenerator, and, in the presence of a magnetic field, the first ensembleof color centers emit a first fluorescence signal representing the firstresonance. The circuitry (e.g., lock-in amplifier) is used tofrequency-lock the first microwave signal to the first resonance basedon the first fluorescence signal. The processor is used to determine anamplitude and/or direction of the magnetic field based on a frequency ofthe first microwave signal.

In some instances, the solid-state host of the vector magnetometercomprises a second ensemble of color centers oriented along a secondcrystal axis of the solid-state host and exhibiting a second resonanceand a third ensemble of color centers oriented along a third crystalaxis of the solid-state host and exhibiting a third resonance in thepresence of the magnetic field. And the circuitry is configured tofrequency-lock a second microwave signal to the second resonance and tofrequency-lock a third microwave signal to the third resonance, suchthat the processor determines the amplitude and/or direction of themagnetic field based on a frequency of the second microwave signal and afrequency of the third microwave signal.

An example method of measuring the magnitude and direction of a magneticfield using the vector magnetometer can comprise frequency-locking thefirst microwave signal to the first resonance and determining anamplitude and/or direction of the magnetic field based on a frequency ofthe first microwave signal. Some other example methods can includefrequency-locking several microwave signals to several resonancesexhibited by the ensemble of color centers (e.g., a second or a thirdmicrowave signal frequency locked to the second or third resonance,etc.) and determining the amplitude and/or direction of the magneticfield based on a frequencies of the several microwave signals.

The first resonance exhibited by the ensemble of color centers canrepresent a first energy-level transition, such as a first hyperfinetransition. The second resonance can represent a second energy leveldifferent from the first energy-level transition, which can also be ahyperfine transition. Similarly, the several resonances with thecorresponding several microwave signals can each represent energy-leveltransitions that can be hyperfine transitions.

The processor can be configured to determine the amplitude and/ordirection of the magnetic field based on the frequencies of the severalmicrowave signals corresponding to the several resonances of theensemble of color centers. For example, by taking a difference betweenthe frequency of the first microwave signal and the frequency of thesecond microwave signal. The processor can determine the amplitude ofmagnetic field with a dynamic range of about 360 μT to about 100,000 μT.

A multi-channel frequency-locking NV magnetometer tied to fundamentalconstants and robust against phenomenological variables such as laserand MW power noise and drift, both of which limit previous lock-in basedNV magnetometers, is especially useful for remote, in situ sensingapplications. As described in detail below, using multiple frequencychannels allows for the simultaneous measurement of more than onetransition of an NV orientation class and consequently the isolation ofthe magnetic field from temperature variations.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1A shows an example of a defect, here a nitrogen-vacancy (NV)center, in a crystal lattice.

FIG. 1B depicts a generalized energy level diagram showing the statetransitions of an NV center from the ground state to an excited stateand back through radiative and non-radiative pathways. The insets showthe splitting of the degenerate spin states m_(s)=±1 in the presence ofa magnetic field along the NV symmetry axis. The degenerate m_(s)=±1states split in the presence of magnetic field B_(NV) with an energylevel separation 2μ_(B)g_(e)B_(NV). Note that other defect centers canalso show a similar effect of energy levels shifting in the presence ofa magnetic field.

FIG. 1C shows a magnified view of the ground state of the energy leveldiagram in FIG. 1B while also including an example of hyperfine statesfor ¹⁴NV.

FIG. 2A shows a tetrahedral frame, which can correspond to an NV-diamondtetrahedral coordinate frame. The thin black arrows indicate possibleorientations of, e.g., NV centers. For an external field oriented nearthe gray arrow, the sub-ensemble of NV centers aligned along the grayarrow would experience the largest energy shift.

FIG. 2B shows an example ODMR spectrum when NV centers of four differentorientations are subjected to an applied magnetic field B that causesdifferent Zeeman splittings (1 to 4) for each of the four possible NVorientation classes.

FIG. 3 shows an example method of measuring magnetic fields using asequential vector magnetometer based on the spin states of an ensembleof solid-state defects.

FIG. 4A shows a block diagram of an example apparatus with physics,electronics and data processing units used to measure magnetic fieldvectors sensed by an ensemble of color centers in a solid-state host.

FIG. 4B shows a schematic of a linearized model of feedback control inan example magnetometer.

FIG. 5 shows an example compact magnetometer with a solid-state hostcontaining many color centers.

FIG. 6A shows an example experimental error spectrum illustratinghyperfine transitions of four NV orientations.

FIG. 6B shows frequency shifts experimentally measured from magneticresonance of ensembles of NV centers in diamond, which has a tetrahedrallattice structure, to magnetic fields applied along the x, y, and zareas in a Cartesian frame of reference in an example experiment.

FIG. 6C shows an example of magnetic field measurements reconstructedfrom frequency shift measurements in FIG. 6B with color centers in asolid state host. The measured field information is transformed into aCartesian reference frame labeled along x, y, and z.

FIG. 7A shows an example of experimentally measured magnetic fieldprojections as a function of incident laser light, illustrating thescale factor-free nature of measurements.

FIG. 7B shows an example experimental measurement of magnetic fieldprojection as a function of time, with insets corresponding tomeasurements of the NV resonance frequencies measured simultaneously,highlighting the dynamic range of an example magnetometer, which is alsoinsensitive to temperature effects.

FIG. 7C shows the noise spectrum of an example magnetic fieldmeasurement, illustrating the magnetic sensitivity of the measurement.

FIGS. 8A and 8B show example ODMR spectra resulting from hyperfinetransitions in the presence of a magnetic field, from ensembles of twodifferent types of NV centers, namely ¹⁴NV and ¹⁵NV.

FIGS. 9A and 9B are example plots of noise spectra, showing shot-noiselimited sensitivity in one channel of an amplifier in an examplemagnetometer apparatus, where the photodetector is powered by anexternal power supply or by a battery, respectively.

FIG. 10A shows a plot of noise measurements as a function of modulationfrequency of an example magnetometer system.

FIG. 10B shows an example of the responsivity of the NV system to noiseas a function of modulation frequency of an example magnetometerapparatus.

FIG. 10C shows an example plot of sensitivity to noise as a function ofmodulation frequency of an example magnetometer system, indicating anoptimum subset of modulation frequencies for operation.

FIG. 11 shows an example signal path for a single-frequency locked loop.

FIG. 12 shows an example optically detected magnetic resonance (ODMR)spectrum of an NV electronic spin in the presence of an applied magneticfield B_(NV). The microwave signal f_(mod) is generated by modulatingf_(LO) at frequency f_(ref) with depth f_(dev). The demodulated lock-insignal is shown for f_(LO) near an NV resonance illustrating that thelock-in signal is zero when f_(LO) is directly on resonance.

FIG. 13 is an example plot of lock-in signal noise as a function ofmodulation frequency. Note that above 4 kHz, the noise is shot-noiselimited whereas below 4 kHz, 1/f technical noise dominates.

FIG. 14 is an example plot of NV system responsivity as a function ofmodulation frequency. Note that the responsivity and subsequently thesensitivity degrades for higher modulation frequencies.

FIG. 15 is an example plot of magnetic sensitivity as a function ofmodulation frequency, indicating the limited range of modulationfrequencies that give optimum magnetic sensitivity.

FIG. 16 is an example of difference in contrast (middle plots) andlock-in signal illustrating responsivity (right plots) of performingmicrowave driving without sideband modulation (top plots) and withsideband modulation tuned to the NV hyperfine splitting (bottom plots).

FIG. 17 is an illustration of driving different hyperfine transitions ofan NV resonance in an NV ensemble.

FIGS. 18A and 18B are example illustrations of how a single pair of NVresonance frequencies (2.732481 GHz, 3.061399 GHz] can yield a range ofextracted values for temperature, magnetic field direction, and magneticfield magnitude.

FIG. 19 is a diagram of a bulk diamond crystal containing many unitcells to illustrate the four possible NV orientation classes, examplesof each are indicated by arrows. Carbon atoms are black, substitutionalnitrogen atoms are blue, and vacancies are white; the NV symmetry axisis shown in red. Note that each orientation class comprises twoorientations that have the same symmetry axis but where the relativepositions of the N and V are switched.

FIG. 20 is a schematic diagram of NV hyperfine structure when thesubstitutional nitrogen is the ¹⁴N isotope of nitrogen (left) and whenthe substitutional nitrogen is the ¹⁵N isotope of nitrogen (right),including the dipole-allowed transitions between the hyperfine states.

FIGS. 21A-21D are noise spectra extracted from simultaneous 8-channelfrequency-locking measurements to illustrate the magnetic sensitivity ofeach of the four possible NV orientations. The shot-noise-limitedexpected sensitivity and sensitivity as measured over a frequency rangebelow the 10 Hz roll-off of the lock-in amplifier and/orfrequency-locking feedback controller are shown for each NV orientationclass.

FIGS. 22A and 22B are plots illustrating measurements of a varyingmagnetic field using a commercial fluxgate magnetometer and an NVmagnetometer, respectively, for comparison. Note that the magnetometerswere placed in proximity to each other but not co-located; however, themagnetic field experienced by both sensors is expected to be comparablesince changes to the field were induced by moving large magnetic objectssituated far away from the sensors.

FIG. 23 shows an example method of measuring magnetic fields using asequential vector magnetometer based on the spin states of an ensembleof solid-state defects.

DETAILED DESCRIPTION Introduction

In recent years, solid-state magnetometers and in particular NV-diamondmagnetometers have shown utility in several modalities: as electricfield sensors of both alternating current (AC) and direct current (DC)signals, as well as performing measurements of magnetic field through,for example, high-resolution scanning of a single NV center,simultaneous wide-field imaging of an NV ensemble located in a thinlayer near the surface of a diamond, and high-sensitivity measurementswith a large NV ensemble in bulk diamond, with demonstrated magneticsensitivity down to <1 pT/√{square root over (Hz)}. Due to the manyuseful properties of both NV centers and diamond, NV-based magnetometerspromise to rival the sensitivity and long-term stability of atomicmagnetometers, while possessing a number of additional advantages, suchas intrinsic vector capability and a compact, solid-state package, notinherent to atomic magnetometers.

One approach to sensing magnetic fields with NV centers uses opticallydetected magnetic resonance (ODMR) measurements to determine the NVground state transition frequencies, which experience Zeeman splittingas a function of the applied magnetic field and are scaled only byfundamental constants, such as the spin, Plank's constant, Bohrmagneton, etc. However, some versions of current NV magnetometers thatemploy this method rely on measurements of NV fluorescence intensity andare therefore susceptible to noise in the optical and microwaveexcitation sources used to perform ODMR, while others performmeasurements only on single resonances of single NV centers and aretherefore both susceptible to noise in temperature as well as incapableof full vector measurements.

In contrast, an inventive NV magnetometer uses a closed-loop,frequency-locking scheme to directly measure multiple Zeeman-split NVresonance frequencies in an NV ensemble, simultaneously. There are anumber of significant advantages of this technique over open-loopmethods: (1) the elimination of steady-state dependence on variableparameters, such as NV resonance contrast and linewidth, resulting in amore robust measurement; and (2) an increase in the dynamic range to ≥10mT, corresponding to a factor of about 1,000 improvement. Furthermore,by using this frequency-locking technique on an ensemble of NV centersin diamond to simultaneously detect more than one resonance frequencyand/or rapidly interleave measurements of resonance pairs of NV centersoriented along some or all of the four diamond crystallographic axes, aninventive magnetometer can perform real-time measurements of the fullmagnetic field vector, decoupled from temperature effects. Note that theultra-stable diamond crystal lattice, with even further stabilityattained by averaging over the ensemble, also provides a significantlymore reliable measure of the vector angle when compared to vectormeasurements formed from three orthogonal scalar magnetometers, whichare limited by machining tolerances.

Briefly, the magnetometer functions by reporting the spin state of NVcenters embedded in a diamond crystal host. Ensembles of NV centersabsorb optical and microwave energy in order to transition betweenenergy levels, while simultaneously emitting photoluminescence. Theintensity of emitted light can be used to identify the average spinstate of an ensemble of NV centers given that transitions betweencertain states are preferably non-radiative compared to transitionsbetween others. The resonance frequencies corresponding to thetransitions of NV ensembles are affected by the presence of a magneticfield through the Zeeman Effect. By using the optical readout in aclosed-loop manner, an inventive magnetometer locks microwave signals tothe transitions, thereby directly measuring the NV resonancefrequencies. NV centers of a particular orientation can be used toextract magnitude and direction information of the local magnetic fieldby simultaneously measuring the pair of resonance frequencies associatedwith that particular NV orientation. Further, NV ensembles of differentorientation classes are differently affected by a magnetic field of agiven direction. This orientation dependence can be used to measure theamplitude of the magnetic field perceived by each orientation class andthus to extract full vector information of the magnetic field.Alternatively, as discussed later, a subset of at minimum four NVresonances is sufficient to extract the full magnetic field vector.

Without being bound or limited by any particular theory, thisapplication discloses a theory of operation and examples of methods andapparatus for measuring and quantifying transitions between electronicstates (sensitive to magnetic fields) of an ensemble of nitrogen-vacancy(NV) centers. It includes example experimental measurements to determinethe magnitude and direction of an external magnetic field or anintrinsic magnetic field under ambient temperature. An external magneticfield may be an ambient magnetic field in which the magnetometerapparatus is placed, or in some instances, a static or variable magneticfield may be applied using a suitable mechanism. For example, Helmholtzcoils or permanent magnets can be used to deliver or apply a magneticfield. In some instances, the external magnetic field can also beintrinsic to a material. In such instances, the material to be examinedcan be embedded with a crystal host comprising of NV centers, and theembedded NV centers can be used to measure and report the local magneticfield within the material. Note also that many of the techniques,operational modes, and processes discussed in this patent applicationcan be generally applied non-spherically-symmetric point defects insolid-state systems.

In summary, the present application describes an implementation of amulti-channel frequency-locking NV magnetometer, with long-term stablemagnetic field measurements that are tied to fundamental constants,robust against phenomenological variables such as laser and MW powernoise and drift, and overcome the limited dynamic range of some of thepreviously demonstrated NV magnetometers. The magnetometer systemdescribed in the present application includes simultaneous measurementof more than one resonance frequency of NV center ensembles, allowingfor the extraction of the full magnetic field vector, decoupled from theeffects of temperature (e.g., variations). In several modes ofoperation, the full magnetic field vector and temperature informationcan be fully extracted in real-time from simultaneous measurements ofmultiple NV resonances without systematic errors or 1/f noise behaviorsuch as are observed in sequential measurements. This is especiallyvaluable for remote, and in situ sensing applications, particularly fora device operating in a moving and/or rotating ambient magnetic fieldand/or operating on a moving and/or rotating platform in an ambientmagnetic field. In these situations, noise due to moving and/or rotatingmagnetic fields and/or platforms is correlated between magnetic fieldprojections along different vector axes and can therefore their effectscan be removed in processing. Note also that additional robustnessagainst moving and/or rotating ambient magnetic fields and/or platformsmay be afforded by the overdetermined nature of the NV-diamondtetrahedral geometry.

Nitrogen-Vacancy Centers in Diamond

A crystalline solid has a structure that comprises repeating units ofatoms or molecules over lattice points. Imperfections in thisarrangement can occur as point defects in the crystal structure, e.g.coinciding with given lattice points. Types of point defects in crystalhosts can include vacancy defects where lattice sites are left vacant,substitutional defects where the atom or molecule at a given latticesite is replaced with a different atom or molecule, and interstitialdefects where the atom or molecule is between lattice cites. These pointdefects result in a redistribution of charge densities and otherproperties, associated with the lattice site. Crystal hosts can alsoinclude point defect complexes formed by defects on adjacent latticesites. For example, in diamond, which is generally composed of carbonatoms in a tetrahedral crystal lattice, the nitrogen-vacancy (NV) defectcomprises a substitutional nitrogen replacing a carbon atom and avacancy, or missing carbon atom, on adjacent lattice sites.

The NV center can be incorporated in the diamond crystal with highdefect density, allowing for a very large ensemble of centers to occupya very compact volume, useful for a compact sensor, discussed in a latersection of this patent application. As a specific example, the crystalhost can be bulk diamond 100 containing nitrogen vacancy (NV) centers asillustrated in FIG. 1A. The NV center 102 shown along one crystaldirection in the diamond host 100 comprises a substitutional nitrogenatom (N) in a lattice site adjacent to a vacancy (V) 102 a. The carbonatoms surrounding the NV center are each labelled C. The NV center mayoccupy any of eight possible orientations (four orientation classescontaining symmetric pairs) corresponding to the location of thesubstitutional nitrogen atom with respect to the vacancy. Examples of NVcenters of each orientation class are indicated by green arrows in FIG.19.

An NV center can exist in a negatively-charged state NV⁻ (generallyreferred to simply as NV) or a neutral-charge state NV⁰. An NV center inthe negatively-charged state has an additional electron in addition tothe five dangling bond electrons, one each from the carbon atoms and onepair of electrons between the vacancy and the nitrogen atom. NV⁻ and NV⁰can be optically distinguished by their ZPLs (zero phonon lines) at 637nm and 575 nm, respectively.

The negatively charged NV center has electronic spin S=1 with a tripletground state where m_(S)=0 and degenerate m_(S)=±1 spin sublevelsexperience a D_(gs)≈2.87 GHz zero-field splitting under ambientconditions and no applied fields as shown in FIG. 1B. Under a non-zeromagnetic field, the m_(S)=±1 levels are no longer degenerate as shown inthe inset in FIG. 1B. As a specific example, in the presence of amagnetic field B_(NV) oriented along the NV symmetry axis, the m_(S)=±1spin sublevels experience Zeeman splitting given by 2γB_(NV). Note thatthis splitting is proportional to the magnitude of the magnetic field,scaled by fundamental constants γ=2μ_(B) g_(e)≈2.8 MHz/G (FIG. 1B).

More generally, the ground-state energy levels experience furthermodification in the presence of local strain, electric field, andmagnetic fields as approximated in the following NV Hamiltonian:

$\begin{matrix}{H_{gs} = {{\left( {{hD}_{gs} + {d_{gs}^{\parallel}\prod\limits_{s}}} \right)\left\lbrack {S_{z}^{2} - {\frac{1}{3}{S\left( {S + 1} \right)}}} \right\rbrack} + {\mu_{B}g_{c}{S \cdot B}} - {d_{gs}^{\bot}\left\lbrack {{\prod\limits_{x}\left( {{S_{x}S_{y}} + {S_{y}S_{x}}} \right)} + {\prod\limits_{y}\left( {S_{x}^{2} - S_{y}^{2}} \right)}} \right\rbrack}}} & (1)\end{matrix}$

Additional terms in the Hamiltonian provide increased accuracy.

Hyperfine interactions between the NV electronic spin and the nuclearspin of the nitrogen atom that makes up the NV cause further splittingin the spin sublevels of the ground-state. This hyperfine structurealong with the allowed dipole transitions are shown for the two mostcommon nitrogen isotopes (¹⁴N, =1 and ¹⁵N, I=½) in FIG. 1C (¹⁴N) andFIG. 20 (¹⁴N and ¹⁵N). FIGS. 8A and 8B show the ODMR spectra (describedin greater detail below) of NV ensemble measurements, taken in thepresence of an arbitrary external magnetic field, using vectormagnetometers with crystal hosts in which the nitrogen atoms arepredominantly the ¹⁴N isotope (FIG. 8A) or predominantly the ¹⁵N isotope(FIG. 8B), respectively. FIGS. 8A and 8B illustrate the effect ofdifferent hyperfine states in the NV hyperfine structure.

A number of particularly useful properties of the NV center originatefrom the system's behavior under optical excitation. FIG. 1B shows aschematic of the energy level diagram when an exemplary NV center isoptically excited to transition from the ground (triplet) state |g> toan excited triplet state |e>, using excitation light of 532 nm (greenarrow). In particular, excitation via optical photons containing energyequal to or greater than the NV 637-nm zero-phonon line causes the NVcenter to transition from the ground state |g> to the excited state |e>,where the excited state |e> is also a triplet state with threespin-sublevels denoted by m_(S)=0, m_(S)=+1, and m_(S)=−1. One possiblerelaxation pathway for the excited NV center to return to the groundstate |g> is with the emission of a photon, with wavelength typically inthe range of 637-800 nm. That is, the transition from the excited stateto the ground state can be effected by emitting a photon whose energycorresponds to the energy gap between the energy levels of thetransition, for the zero-phonon line, or the energy gap minus the energyof a phonon in the case of the phonon sidebands. For the most part thisoptical excitation and subsequent fluorescence process maintains thespin state of the NV center.

Another relaxation pathway for the NV to transition from the excitedstate |e> to the ground state |g> is via intermediate states |s>containing a singlet spin state m_(S)=0, as depicted in FIG. 1B. Thisalternative path involves little to no visible photon emission, beingmostly non-radiative. Further, the fine structure levels m_(s)=±1 in theexcited state generally have a higher likelihood of taking thenon-radiative path over the excited state m_(S)=0 sublevel, as indicatedby the solid and dotted arrows in FIG. 1B.

Due to the higher likelihood of the transition from excited m_(s)=±1 viathe non-radiative path, the intensity of optical emission can be used todetermine the spin state of one or more NV centers. That is, the moreNVs in the m_(s)=±1 instead of the m_(s)=0 state, the lower the averagefluorescence intensity. Consequently, by measuring the NV fluorescenceintensity, the spin state of a single NV center or the average spinstate of an ensemble of NV centers can be extracted. Additionally, theunequal transition probability rates between the m_(S)=0 and m_(S)=+1sublevels in the excited state, to the singlet state, to the m_(S)=0 andm_(S)=±1 sublevels in the ground state result in enriched population ofthe m_(S)=0 spin sublevel under prolonged optical excitation.

To summarize, the dynamics of the NV system under optical excitationyield two significant and useful behaviors: (1) the NV fluorescenceintensity reflects and thus provides a measure of the NV spin state and(2) the NV is eventually polarized into the m_(S)=0 ground state underoptical excitation. The timescale for the polarization into the m_(S)=0ground state to saturate is a function of the optical excitationintensity, capable of reaching full polarization within several hundrednanoseconds (˜300 ns) at the optical saturation intensity of the NVcenter.

NV-Diamond Magnetometry

As mentioned previously, one approach to performing magnetic sensingwith NV centers is through optically detected magnetic resonance (ODMR)measurements, which take advantage of the behavior of the NV systemunder optical excitation to measure the transition frequencies betweenthe NV electronic energy levels and the dependence of the NV electronicenergy levels on magnetic fields. ODMR measurements also take advantageof the fact that microwave radiation that is resonant with, i.e. equalto the difference between, certain electronic energy levels can causetransitions between those electronic energy levels.

In the NV spin system, MW excitation that is resonant with so-calleddipole-allowed transitions, indicated by the arrows in FIG. 1C for ¹⁴NVand in FIG. 20 for ¹⁴NV and ¹⁵NV, can cause the NV spin state totransition between the two energy levels connected by the transition.Note, also, that under certain circumstances, resonant MW excitation candrive even the so-called “forbidden” transitions.

Applying continuous optical and MW excitation while sweeping thefrequency of the MW drive can yield an ODMR spectrum from which thetransition frequencies of the NV system can be extracted. When the MWdrive frequency is not resonant with any transitions, the opticalexcitation pumps the NV spin into the m_(S)=0 state and the NVfluoresces with maximum intensity. However, when the MW is resonant witha transition (generally an m_(S)=0-1 or m_(S)=0→+1 transition), some ofthe NV population is cycled into the m_(S)=±1 state resulting in areduced NV fluorescence intensity. When the MW drive frequency is swept,these reductions in fluorescence intensity corresponding to allowedtransition frequencies manifest as resonance dips. In this way, we canperform spectroscopy to determine the NV ground-state transitionfrequencies and thus extract, e.g., the magnetic field (among otherphysical phenomena). Given the D_(g)s 2.87 GHz zero-field-splitting ofthe NV center, a typical MW frequency sweep range for an NV ODMRspectrum may be from about 2.65 GHz to about 3.1 GHz. Other instances ofmeasuring an NV ODMR spectrum can involve sweeping the MW frequencyacross other suitable or appropriate ranges, as desired.

A collection of NV centers, such as the NV 102, within the bulk diamondhost 100 is referred to as an ensemble of NV centers and can be used tomeasure the magnitude and/or direction of magnetic, electric, and strainfields applied to the diamond host 302. Recall that due to thetetrahedral structure of the diamond crystal, NV centers may occupy anyof eight possible orientations, corresponding to four orientationclasses (where each orientation class contains two orientations with thesame symmetry axis but where the N and V are in opposite lattice sites).Each NV center (particularly those in different orientation classes) mayexperience a different local field and hence exhibit a different changein resonance frequency, which can be measured using ODMR through changesin the average emitted photoluminescence due to MW driving of NV spinstate transitions.

Later in this patent application, we will describe a family of inventivetechniques that allow for the simultaneous measurement of multiple NVresonances across multiple NV orientation classes in an ensemble ratherthan as a (sequential) sweep. Using this family of techniques, each NVorientation class can simultaneously report on the average spin-state inthat orientation and therefore on the magnetic field, electric field, orother physical phenomena perceived in that orientation. In particular,each NV orientation class is generally most sensitive to magnetic fieldprojections along their symmetry axis. Consequently, measuring an NVensemble, which contains NVs in all four orientation classes,automatically yields full magnetic field vector information, where thevector axes are tied to the stable diamond lattice.

An example NV ensemble ODMR spectrum is shown in FIG. 2B, with anexternal magnetic field producing different magnetic field projectionsalong each of the four symmetry axes corresponding to the four NVorientation classes. A diagram of a tetrahedron illustrating the NVDiamond tetrahedral coordinate frame of reference is shown in FIG. 2A,with the four possible orientations of the NV symmetry axis (dark thinarrows) are superimposed by the direction of magnetic field (thick lightarrow). That is, one can picture the tetrahedron as a diagram of the NVcenter where the substitutional nitrogen occupies the dark centrallattice site and the vacancy may occupy any one of the four cornerlattice sites.

Note that the example NV ensemble ODMR spectrum of FIG. 2B does notcorrespond to the magnetic field direction indicated in the diagram ofFIG. 2A. The magnetic field applied to an NV ensemble to yield the ODMRspectrum of FIG. 2B has unequal projections along each of the foursymmetry axes corresponding to the four NV orientation classes, with theresonance pairs corresponding to each of the four NV orientationslabeled by horizontal arrows and indicated with labels 1-4 in order ofhighest magnetic field projection along the NV symmetry axes (1) tolowest magnetic field projection along the NV symmetry axis (4). Notethat due to the separation in the energy level, transitions to them_(s)=+1 and −1 levels can be distinguished in an ODMR measurement bytheir different frequencies corresponding to the different energyseparations.

Note that the example ODMR spectra in FIG. 2B are obtained by sweepingthe MW frequency and are therefore sequential rather than simultaneousmeasurements of each of the NV resonances. However, only by measuringmultiple NV transition frequencies simultaneously can the effects oftemperature and magnetic field, which both shift the NV resonances, bedecoupled to extract the projection of the magnetic field along the NVsymmetry axis. Inventive techniques for performing simultaneousmeasurements on NV ensembles, using some embodiments of the inventivevector magnetometer, are described below in this patent application.

NV-Diamond Magnetic Sensitivity

In a specific example where the readout mechanism of the spin state isoptical, the optimum, shot-noise-limited sensitivity of an NV-basedmagnetometer is dependent on several factors encompassed in the equationbelow:

$\begin{matrix}{\eta \propto {\frac{h}{g_{e}\mu_{B}}\frac{1}{\alpha \sqrt{\beta \; {NT}}}}} & (2)\end{matrix}$

where h, g_(e), and μ_(B) are fundamental constants; α is the contrastof the measurement (related linearly to the responsivity of the NVsystem in lock-in based measurements discussed later in the patentapplication), β is the average number of photons collected per NV, N isthe number of NV sensors, and T for the ODMR measurements describedpreviously is T₂*, inversely related to the linewidth of the NVresonances. These factors are related to the diamond material propertiesand the device implementation, which can both be engineered andoptimized for different measurement modalities.

Various inventive techniques that can be employed to improve contrast a(or NV system responsivity as discussed later), to improve measurementduty cycle, or to weight contributions of measurements from different NVorientation classes depending of various factors (e.g., the degree ofmagnetic field projections along the symmetry axes of each orientationclass), are described herein. The latter two are considerations that arenot encompassed in Equation (2). However, we note that from the numberof NV sensors N, there is a potentially substantial improvement tomagnetic sensitivity from performing measurements on large NV ensembles,such as afforded by the high defect densities that can be incorporatedinto compact solid-state volumes. An alternative way to view theimprovement to magnetic sensitivity afforded by performing ensemblemeasurements is that ensemble measurements yield higher totalfluorescence signal, resulting in higher signal-to-noise ratio and thusallowing shorter measurement times to reach a given minimum detectablefield.

Frequency-Locking Techniques for NV-Diamond Magnetometry Introduction

Previous demonstrations of ODMR-based NV magnetometry measured fullspectra and performed fits to extract the NV resonance frequencies.These methods involve spending a large fraction of the measurement timemonitoring non-information-containing, off-resonance signal and issubsequently prohibitively slow for some applications. Fitting thecurves also introduces latency that may be incompatible with real-timesensing.

More recent demonstrations used lock-in techniques to continuouslymonitor a single resonance on the approximately-linear derivativesection of the spectral feature, from which small resonance frequencyshifts were detected by applying pre-calibrated scale factors. However,this second approach is limited to the approximately linear regime ofthe lock-in signal, resulting in a dynamic range of a few μT.Furthermore, this method is inherently dependent on phenomenologicalvariables instead of a true frequency shift. In particular, the scalefactor is influenced by the NV resonance linewidth and contrast, both ofwhich are affected by optical pump power, microwave power, and detectionefficiency. These variables are different for each device and will alsodrift over time, consequently requiring periodic recalibration.

The present application describes a magnetic field sensing apparatus andimplementation of the apparatus with a closed-loop system that directlylocks the microwave drive field to one or more NV resonances, thusisolating the magnetic field measurement from the phenomenologicalvariables that determine the scale factor of previous lock-in-basedapproaches. This frequency-locking technique is similar to that employedcanonically in atomic Mz type magnetometers previously and, morerecently, demonstrated on single NV centers in diamond. The techniquepresented here is extended to perform measurements on multiple NVresonances simultaneously, to fully decouple temperature from magneticfield, and apply it to an NV ensemble to extract the full magnetic fieldvector, thus demonstrating a capability not inherent to Mz type atomicmagnetometers nor single NV centers.

Single-Channel Frequency-Locking

First, we describe the technique as it applies to a single NV resonanceand address advantages as well as extensions that yield furtheroperational improvements. In later sections, we discuss extensions ofthe technique and associated benefits thereof. FIG. 11 shows a diagramof the signal path of a frequency locked loop for a single NV resonance.A signal generator outputs a microwave signal frequency, f_(LO), whichis tuned to minimize the error on the feedback compensator. Themicrowave signal is modulated at frequency f_(ref) with depth f_(dev) toproduce a time-dependent frequency given by:

f _(mod)(t)=f _(LO) +f _(dev) cos(2πf _(rel) t)  (3)

While f_(LO) is set to minimize the error signal and f_(dev) is chosento maximize the responsivity of the NV system (that is, how much thelock-in signal changes given a small shift in the NV resonancefrequency), there are some additional considerations that go into theselection of an appropriate f_(ref); these considerations will beelaborated upon shortly. The signal is sent to an antenna which drivesthe NV spin with the modulated microwave field. The fluorescence signalfrom the diamond sample is detected with a photodetector and demodulatedby a lock-in amplifier.

One can set the phase of the lock-in amplifier such that the in-phasecomponent of the lock-in signal is positive (negative) if f_(LO) isslightly below (above) the NV transition frequency, whereas the lock-insignal is zero when f_(LO) is directly on resonance with the NVtransition frequency [see FIG. 12]. Note that optimum choice of phasecan maximize the responsivity of the NV lock-in signal to shifts in theNV resonance (thereby improving magnetic sensitivity); however differentphase settings may be optimum for different situations. A feedbackcompensator, which may include a proportional-integral-derivative (PID)controller, produces an error signal from the lock-in signal. The errorsignal in turn provides feedback to adjust the microwave sourcefrequency f_(LO) and lock it to the center of the NV resonance.

The dynamics of the feedback loop can be analyzed using a linearapproximation of the system. For example, the fluorescence amplitude asa function of time can be described by approximating theLorentzian-shaped NV resonance as a quadratic polynomial (valid forf_(LO)−f₀<σ≈500 kHz):

$\begin{matrix}\begin{matrix}{V_{f\; 1} = {V_{0}\left( {1 - \frac{c}{1 + \left( \frac{f_{mod} - f_{0}}{\sigma} \right)^{2}}} \right)}} \\{\approx {V_{0}\left( {1 - C + {\frac{2C}{\sigma^{2}}\left( {f_{mod} - f_{0}} \right)^{2}}} \right)}}\end{matrix} & (4)\end{matrix}$

where C is the contrast, f₀ is the resonant frequency, and σ is half ofthe full-width half-maximum (FWHM) of the Lorentzian. This responsetranslates the frequency modulation of the microwaves to an amplitudemodulation of the fluorescence signal.

The output of the lock-in amplifier is the fluorescence signal mixedwith the reference (f_(ref)) and low-pass filtered:

$\begin{matrix}{{V_{out}(f)} - {{V_{f\; 1}(f)}*\frac{{\delta \left( {f - f_{ref}} \right)} + {\delta \left( {f + f_{ref}} \right)}}{2}{H(f)}}} & (5)\end{matrix}$

where H(f) is the transfer function of the digital low pass filter.

Combining equations (4) and (5) results in a sum of sinusoids atdifferent harmonics of f_ref convolved with the low-pass filterresponse. The cutoff frequency of the filter can be set at a much lowerfrequency (e.g., 160 Hz) than the reference frequency so that thecontribution from most or all terms except those mixed down to DC arefiltered out, resulting in the approximate output:

$\begin{matrix}{{V_{out}(f)} = {\frac{2{CV}_{0}}{\sigma^{2}}{f_{dev}\left\lbrack {{f_{LO}(f)} - {f_{0}(f)}} \right\rbrack}{H(f)}}} & (6)\end{matrix}$

In the case of a closed-loop system such as the one employed with thefrequency-locking technique, the error (i.e., the lock-in signal) set by[f_(LO)(f)−f₀(f)] is kept small. Consequently, operation of thepresented magnetometer can be modeled as a linearized dynamical system.The system dynamics can be analyzed using a simple control architecture,such as that depicted in FIG. 4B. The model system 450 includes a stage452 described Plant transfer function G(z) and a controller 454 that isshown as a digital integral controller in this particular examplesystem, but may generally include proportional, integral, and/orderivative gain parameters.

The first stage 452 in the control architecture diagram includes anamplification stage 458 of the frequency error [f_(LO) (f)−f₀(f)] signalby the responsivity of the NV system, approximated to be linear as wehave discussed, followed by a low-pass filter stage 456, such as the onediscussed above. Mathematically, this stage is described by a Planttransfer function

$\begin{matrix}{{{G(z)} = {\frac{2{CV}_{0}}{\sigma^{2}}f_{dev}{H(z)}}},} & (7)\end{matrix}$

where H(z) is the transfer function of the low pass filter 456. In thisexample analysis of this example system, we use a digital first orderIIR filter such that

${{H(z)} = \frac{\alpha \; z}{z + \alpha - 1}},$

where

${\alpha = {2{{\pi \left( {f_{c}/f_{s}} \right)}/\left\lbrack {{2{\pi \left( \frac{f_{c}}{f_{s}} \right)}} + 1} \right\rbrack}}},f_{s}$

is the sampling frequency, and f_(c) is the filter cut-off frequency.Note however that in general any implementation of a low pass filter canbe employed.

The Plant transfer function is then given (in this model case) by:

$\begin{matrix}{{G(z)} = {\frac{2{CV}_{0}}{\sigma^{2}}f_{dev}\frac{\alpha \; z}{z + \alpha - 1}}} & (8)\end{matrix}$

From the above equation, we can see that when this system is applied toopen-loop measurements, the gain is proportional to a number ofexperimental parameters: signal contrast (C), fluorescence intensity(V₀), and resonance linewidth (σ). These parameters are susceptible tofluctuations in both laser intensity and microwave power, which degradethe accuracy of the open-loop sensor.

The addition of a feedback loop allows for tracking the frequency of theresonance without this dependence on non-fundamental experimentalparameters. In this example analysis, we implement the feedback loop viaa digital integral controller 454, with open loop transfer functiongiven by:

$\begin{matrix}{{C({az})} = \frac{K_{I}z}{z - 1}} & (9)\end{matrix}$

Note however that the feedback loop controller may be implemented moregenerally, e.g., to include proportional, integral, and/or derivativegain parameters. The open loop transfer function is then:

$\begin{matrix}{{{L(z)} = {{{G(z)}{C(z)}} = {\frac{2{CV}_{0}}{\sigma^{2}}f_{dev}K_{I}\frac{\alpha \; z}{z + \alpha - 1}\frac{z}{z - 1}}}},} & (10)\end{matrix}$

and the closed loop transfer function from

${{F_{0}(z)}\mspace{14mu} {to}\mspace{14mu} {F_{LO}(z)}\mspace{14mu} {is}\mspace{14mu} {T(z)}} = {\frac{L(z)}{1 + {L(z)}}.}$

Note that a simple integrator controller increases the open-loop DC gainto eliminate steady-state error. From Eq. (10), the open loop gain canbe increased in multiple ways: through an increase in contrast (C),fluorescence amplitude (V₀), or integrator gain (K_(I)), a decrease inlinewidth (σ); or a combination thereof (note that increasing f_(dev) bytoo much would break the original condition for the validity of thislinear approximation). However, different implementations of thefeedback controller will yield different advantages and disadvantagesand may be tailored for specific applications.

In this example of employing a digital integrator controller with gainK_(I)), the resulting closed loop transfer function is given by

$\begin{matrix}{{{T(z)} = \frac{{G(z)}{C(z)}}{1 + {{G(z)}{C(z)}}}},} & (11)\end{matrix}$

For large gain values, the closed loop transfer function isapproximately independent of experimental parameters and is thus robustagainst laser and microwave intensity noise. Furthermore, invoking thefinal value theorem with a step input, the previous equation can beshown to have no steady-state error. The closed loop system accuratelytracks the resonant frequency in steady-state, and only the dynamics ofthe transient response are influenced by non-fundamental experimentalparameters.

While a scale-factor-free measurement of the magnetic field tied only tofundamental constants and subsequently immune to technical noise, suchas laser intensity and microwave power fluctuations, is one of the majoradvantages of the frequency-locking technique described here, there area number of additional advantages to the technique. Fast feedbackcapability allows for rapid, real-time measurements, limited only by thelock-in filtering and frequency-locking response time. The magneticsensitivity of this technique is as good as or better than that of otherNV ODMR techniques, containing inherently no measurement dead time andbeing generally limited by the shot noise limit indicated in Equation(2) but ultimately potentially by the spin projection limit.Furthermore, by virtue of locking to the NV resonance frequency, themeasurement is always performed in the approximately linear regime ofthe lock-in signal, not only making it possible to access the fulldynamic range of the NV system (compared to previous open-looptechniques) but additionally enabling maximal sensitivity over the fulldynamic range of the NV center.

Note that the full dynamic range inherent to the NV center spans fromzero external field to >1 tesla. There are some technical considerationsregarding fluorescence suppression due to spin state level mixing atparticular field strengths related to when certain energy levels wouldcross each other; however, these issues may be overcome, for example, bytaking advantage of the over-determined system to use information fromdifferent NV orientation classes or even by methods of non-fluorescentreadout. The overdetermined system and information from other NVorientation classes can also help overcome potential ambiguities whereindividual NV resonances cross and/or are degenerate at crossoverpoints. Consequently, the dynamic range of an NV magnetometer employingthe frequency-locking technique may reach the full >1 tesla dynamicrange inherent to the NV system.

As mentioned above, there are some technical considerations involved inthe section of the modulation frequency f_(ref). One consideration isthe noise due to various technical components of the measurementapparatus; in particular, the laser and other electronic systems haveunavoidable 1/f noise. Modulating at frequencies where this noise isdominant increases the noise floor of the measurement beyond thefundamental shot-noise limit and subsequently reduces the sensitivity ofthe measurement [See FIG. 13]. For this reason, modulation frequenciesabove the 1/f corner are preferred.

Another consideration is the response of the spin system to the changingmicrowave frequency. The NV center has finite response time to MW drivemodulation, resulting in reduced responsivity at higher modulationfrequencies [see FIG. 14]. Responsivity here is essentially how much thelock-in signal changes in response to a given small change in the NVresonance frequency. The degree of this finite response time effect issomewhat dependent on optical excitation power and can be mitigated byincreasing the optical power. However, this effect makes lowermodulation frequencies preferential.

Dividing the noise from the first consideration mentioned by theresponsivity from the second consideration and then dividing by the NVgyromagnetic ratio γ=28.0249 Hz/nT gives the magnetic sensitivityexpected from taking both considerations into account [see FIG. 15]. Asa result, there is typically a limited range of optimum modulationfrequencies; the upper and lower limits of this optimum modulationfrequency range can be affected by choice of low 1/f noise electronicsand/or higher laser power, but it typically spans from several kHz to10-20 kHz.

FIGS. 10A-10C provide an additional example of performance of the NVsystem under varying modulation frequencies. Note that the plot of noiseas a function of modulation frequency shown in FIG. 10A has highfrequency noise˜6 kHz in addition to the expected 1/f noise. Note howthe noise in FIG. 10A propagates to the plot of expected sensitivity asa function of modulation frequency in FIG. 10C, consequently limitingthe range of modulation frequencies for optimum sensitivity. This secondexample illustrates the importance of minimizing technical noise toallow for greater flexibility in choice of modulation frequency; thiswill become especially relevant for techniques described later involvingsimultaneous driving of multiple frequency-locking channels.

In addition to the inherent advantages and technical details of thefrequency-locking technique discussed thus far, there are severaloptional operational modes that provide further improvements, e.g., tomagnetic sensitivity, that we wish to expand upon here, beforediscussing extending the technique to multiple NV resonances.

First, as mentioned during the discussion of the NV hyperfine structure,while the most common isotope of nitrogen is ¹⁴N (nuclear spin I=1) at99.6% natural abundance, diamond is also commonly synthesized with ¹⁵N(nuclear spin I=½). Due to hyperfine interactions between the NVelectronic spin and the nuclear spin of the nitrogen atom in the NV,each NV resonance may have either three hyperfine transitions (¹⁴N) ortwo hyperfine transitions (¹⁵N). Examples of these two cases for an NVensemble are shown in the ODMR spectra given in FIG. 16, with ¹⁵N shownon the top spectrum and ¹⁴N shown on the bottom spectrum.

Under typical ambient conditions, the hyperfine states are roughlyequally populated; that is, for an NV containing the ¹⁵N isotope, halfof the population is in one hyperfine state and half is in the other,and for an NV containing the ¹⁴N isotope, ⅓ of the population is in eachof the three hyperfine states. Consequently, the contrast of a singlehyperfine transition of an NV containing the ¹⁵N isotope isapproximately 50% higher than the contrast of a single hyperfinetransition of an NV containing the ¹⁴N isotope. Recall that contrast(denoted by a in the sensitivity equation given in the NV section above)is linearly related to the magnetic sensitivity, which means that thesensitivity of ¹⁵N-containing NV centers is 50% better than that of¹⁴N-containing NV centers, in the simplest operating mode where thefrequency-locking technique is applied to a single hyperfine transitionof a single NV resonance frequency.

A second advanced technique involves mixing the modulated MW drive withan additional signal whose frequency is equal to that of the NVhyperfine splitting, thus producing sidebands that coincide with theother one or two hyperfine transitions of the addressed NV resonances.In this way, all hyperfine transitions can be driven, rather than one,improving the responsivity of the NV system greatly. This isillustrated, for example, on a ¹⁴N-containing NV resonance in FIG. 16.

Note that incorporating this sideband modulation technique and drivingall hyperfine transitions can recoup the contrast (and subsequentsensitivity) loss due to driving one of three hyperfine transitions of¹⁴NV compared to one of two hyperfine transitions of ¹⁵NV discussedabove. The equalization of the sensitivities of the different nitrogenisotopes allows ¹⁴NV to be employed in situations where it is preferablewithout a loss in sensitivity. In particular, ¹⁴NV has a strongquadrupolar moment that may be useful for certain measurementmodalities, and the ¹⁴N isotope is more naturally abundant and oftencheaper to incorporate into diamond.

In the next four sections, we will expand on the inventiveness ofperforming the frequency-locking technique discussed here on multipletransitions of an NV ensemble. Doing so has not been previouslydemonstrated and is used for measuring the full magnetic vector anddecoupling these magnetic measurements from temperature fluctuations andother technical noise.

Simultaneous 2-Channel Frequency-Locking

Recall that the behavior of the NV resonance frequencies in the presenceof magnetic fields, electric fields, and strain is described by the NVground-state Hamiltonian below:

$\begin{matrix}{{H_{gs} = {{\left( {{hD}_{gs} + {d_{gs}^{\parallel}\Pi_{g}}} \right)\left\lbrack {S_{g}^{2} - {\frac{1}{3}{S\left( {S + 1} \right)}}} \right\rbrack} +}}{{\mu_{B}g_{e}{S \cdot B}} - {d_{gs}^{\bot}\left\lbrack {{\Pi_{x}\left( {{S_{x}S_{y}} + {S_{y}S_{x}}} \right)} + {\Pi_{\gamma}\left( {S_{x}^{2} - S_{y}^{2}} \right)}} \right\rbrack}}} & (12)\end{matrix}$

where the (Π_(X), Π_(Y), Π_(Z)) terms correspond to electric field andstrain. An NV-diamond system employed as a magnetometer can beconfigured to be negligibly sensitive to electric fields, e.g., with theapplication of a magnetic field with non-zero projection along the NVsymmetry axis. Also, in the course of typical operation, the strain ofthe diamond host does not change. Consequently, we typically neglect theelectric field and strain terms of the Hamiltonian for an NV-diamondsystem operating as a magnetometer.

The simplified Hamiltonian is then:

H _(gs) =h D _(gs) [S _(Z) ²−(S/3)(S+1)]+μ_(B) g _(e) S●B  (13)

In the limit where the magnetic field is mostly aligned to the NVsymmetry axis, the Hamiltonian can be approximated as follows:

H _(gs) ≈h D _(gs) [Sz ²−(S/3)(S+1)]+μ_(B) g _(e) B _(Z) S _(Z)  (14)

However, the zero-field splitting D_(gs) varies with temperature withlinear dependence near room temperature (at minimum over the temperaturerange of 280K-330K), given approximately by

D _(gs) ≈D _(gs)(T=0K)+βγΔT  (15)

where D_(gs)≈2.87 GHz is the zero field splitting at 300 K, β_(γ)≈−74kHz/K near room temperature, and ΔT is the temperature offset from 300 Kduring the measurement.

From this approximation and subsequent analysis, we see that the NVground-state energy levels and thus the NV resonance frequencies aredependent on both magnetic field and temperature. In particular, themagnetic field mostly increases the splitting between the two NVresonances, while the temperature globally shifts both resonances eitherup or down. Because of these interrelated effects, knowledge of one NVresonance frequency is not sufficient information to accurately extractthe magnetic field, even in the simple axial magnetic fieldapproximation. Instead, this case of a single NV orientation class in anapproximately axial magnetic field uses knowledge of both the upper andlower NV resonance frequencies.

The basic, single-channel frequency-locking technique described in theprevious section can be extended by adding a second channel to measure asecond NV resonance in a manner that is generally straightforward. Thesecond channel is centered around a carrier frequency f_(LO,2) set tothe chosen hyperfine transition of the NV resonance to minimize theerror signal. The second channel is modulated at a distinct modulationfrequency f_(ref,2)≠f_(ref,1) and the NV fluorescence signal isdemodulated at that same frequency, yielding independent lock-in signalsfor the two NV resonances from a single optical measurement. In thisway, adding a second channel requires no additional hardware ormeasurement time and involves minimal additional processing overhead.However, note that due to the shared optical readout channel,simultaneously driving multiple NV resonances may require morecomplicated demodulation and filtering to avoid crosstalk between themultiple modulation frequencies. Furthermore, the response bandwidth andresponsivity (related to sensitivity) of a given channel degrades atmodulation frequencies outside of the optimal sensitivity rangediscussed in the previous section. Consequently, the modulationfrequencies should not be chosen to be arbitrarily far apart in order toreduce crosstalk; instead, it is important to choose a second modulationfrequency that is far enough away from the first modulation frequency toavoid unintended cross-talk and intermodulation interference, whilemaking sure both modulation frequencies are still in the range foroptimal sensitivity.

Note that driving two transitions that both involve the same spin statecan result in depopulation and first order coherent driving effects,which affect the lineshape deleteriously. This undesired effect can beavoided by driving different hyperfine transitions in the lowerresonance compared to the upper resonances; due to the 2+ hyperfinetransitions inherent to the NV regardless of nitrogen isotope, thissolution is always possible. One example possibility for drivingdifferent hyperfine transitions of the upper and lower NV resonances isshown in FIG. 17, with the solid lines in the lower resonancescorresponding to m_(S)=0→−1 indicating the m_(I)=−1 hyperfine state andthe dashed lines in the upper resonances corresponding to m_(S)=0→+1indicating the m_(I)=0 hyperfine state. Note that this method may not becompatible with the sideband modulation technique discussed previously,since the sideband technique drives all hyperfine transitions of an NVresonance.

Additionally, while measurement of both upper and lower resonances hasbeen demonstrated previously on a single NV center by switching asingle-channel frequency-locking implementation between the tworesonances, the non-simultaneity of the measurement introducessystematic error for temperature and magnetic field variations that arenot significantly slower than the switching frequency. Furthermore, theswitching frequency introduces noise that degrades the sensitivity ofthe magnetometer with a 1/f characteristic.

Employing the generally straightforward extension described in thissection to the basic frequency-locking technique allows for thesimultaneous measurement of the upper and lower resonances of a singleNV orientation class with minimal additional hardware and processingoverhead, and is appropriate for accurately decoupling measurements ofmagnetic field from temperature fluctuations over all timescales, aswell as from technical noise as afforded by the frequency-lockingtechnique in general. Making magnetic field measurements independent oftemperature further allows for a more robust sensor that has no need fortemperature control.

Sequential Vector Magnetometry Via Simultaneous 2-ChannelFrequency-Locking

As discussed in the previous section, simultaneous measurement of theupper and lower resonances of a single NV orientation class allows thelocal magnetic field and temperature experienced by the NV ensemble tobe decoupled. However, for a single NV orientation classes, thisdecoupling is most effective in the regime where either the alignment ofthe magnetic field with respect to the NV symmetry axis or the magneticfield magnitude is known. Outside of this condition, the magnetic fieldvector (comprising magnitude and direction information) and temperatureare once again coupled. FIGS. 18A and 18B illustrate how, for a pair ofmeasured resonance frequencies, the assumption of different temperaturesyields different extracted values for the angle θ between the magneticfield vector and the NV symmetry axis and magnetic field magnitude.Mathematically, this problem makes intuitive sense, as there are threeunknowns and only two knowns. Thus, the temperature and the full vectorinformation of an arbitrary magnetic cannot be generally extracted froma single NV orientation class.

An NV ensemble in bulk diamond, however, naturally includes four NVorientation classes, with symmetry axes along four well-known, stabledirections. In the presence of a magnetic field in an arbitrarydirection, the four orientation classes will correspond to fourdifferent pairs of resonance frequencies, with several importantcharacteristics: (1) the temperature affects each NV orientation classthe same, thus shifting the zero-field splitting by the same amount; (2)each NV orientation class will experience the same magnetic fieldmagnitude but, generally, different axial and transverse components ofthe field with respect to the individual NV axes; and (3) the symmetryaxes corresponding to each NV orientation class are known with respectto each other and set by the inherent, stable diamond lattice. As aresult, performing measurements on multiple NV orientation classes,naturally occurring in a large NV ensemble, allows for the fullextraction of the full magnetic field vector, decoupled from temperatureeffects, for a general magnetic field orientation.

One approach to performing measurements on multiple NV orientationclasses is a simple extension of the simultaneous 2-resonancefrequency-locking technique described in the previous section:specifically, simultaneously locking to the upper and lower resonancesof a single NV orientation class at a time and rapidly cycling throughmultiple NV classes. To implement this sequential vector magnetometryapproach, one could (1) first perform an ODMR sweep to locate thepositions of all 8 NV resonances; (2) apply the dual-channelsimultaneous frequency-locking technique on the upper and lowerresonances of one NV orientation class for a short sampling period; (3)using the same dual-channel implementation, change the MW drivefrequencies to lock to the upper and lower resonances of a second NVorientation class for a short sampling period, (4) using the samedual-channel implementation, change the MW drive frequencies to lock tothe upper and lower resonances of a third NV orientation class for ashort sampling period; (5) optionally, using the same dual-channelimplementation, change the MW drive frequencies to lock to the upper andlower resonances of the fourth NV orientation class for a short samplingperiod; before finally (6) returning to the first NV orientation class.

Using this process, we can rapidly cycle between all four (oralternatively a subset of three) NV orientation classes to extractinformation on the magnetic field magnitude and orientation with respectto each of the chosen NV orientation class symmetry axes. Then, usingthe known relative orientation of the symmetry axes with respect to eachother, the full magnetic field vector can be reconstructed andtransformed into a 3-axis Cartesian coordinate frame. Mathematically,this technique can be understood intuitively as measuring 6 or 8 knowns(i.e., NV resonance frequencies) in order to extract 4 unknowns(temperature and three components of the magnetic field vector). Inparticular, only three of the NV orientation classes can be used tofully reconstruct the full magnetic field vector in the 3-axis Cartesiancoordinate frame, and for certain applications, it may be preferable toincrease the overall cycling frequency by only sampling three of thefour NV orientation classes.

However, the tetrahedral geometry of the diamond crystal lattice (andconsequently the NV-based magnetometer's inherent coordinate frame) isan optimally overdetermined system that affords a number of usefuladvantages for vector magnetic sensing, both in measurement accuracy andsensitivity. In particular, with extra information about the magneticfield, one can apply different weights to determine how much certaininformation (e.g., individual resonance frequencies or orientationclasses) contribute to the calculation of the magnetic field vector andtemperature, in order to, e.g., significantly enhance the sensitivity ofthe magnetic measurement. Using real-time processing these weights canbe constantly updated to keep the NV magnetometer in its optimalsensitivity regime.

Note that there is also the option of cycling between 3 NV orientationclasses and real-time updating which NV orientation classes to employ inthe measurement for optimized sensitivity, as a way to take advantage ofthe faster cycling time of the 3-NV-class process while optimizing thesensitivity by choice of “weighting” similar to the 4-NV-class process.Regardless of the specific approach, this technique has the majoradvantage of allowing for the full reconstruction of the magnetic fieldvector, decoupled from temperature and other technical noise as affordedby the basic frequency-locking technique. Furthermore, there is almostno additional overhead in hardware or technical complexity compared tothe basic simultaneous dual-channel frequency-locking techniquedescribed in the previous section.

As a trade-off for the minimal increase in technical complexity, thissequential vector magnetometry technique may experience the samelimitations as discussed previously in the case of sequentialmeasurements of the upper and lower resonance frequencies of a single NVorientation class: changes in the magnetic field and/or temperature thatare faster than the rate of cycling between measurements of different NVorientation classes can result in a systematic error in the extractedmagnetic field and temperature values and may increase the noise floorof the magnetic sensitivity in a 1/f fashion. However, many systematiceffects may be mitigated by continuously randomizing the order andsampling time of the measurements on the different NV orientationclasses.

Simultaneous Vector Magnetometry Via Simultaneous 8-ChannelFrequency-Locking

Another approach to performing measurements on multiple NV orientationclasses that overcomes the systematics of the sequential measurement isanother simple extension of the simultaneous 2-resonancefrequency-locking technique to a simultaneous 8-resonancefrequency-locking. In this case, much as we did for the simultaneousdual-channel case, n channels can be added to measure an n-th NVresonance in a manner that is generally straightforward. The n-thchannel is centered around a carrier frequency f_(LO,n) set to thechosen hyperfine transition of the NV resonance to minimize the errorsignal. The n-th channel is modulated at a distinct modulation frequencyf_(ref,n) that is not equal to the modulation frequency of any otherchannel, and the NV fluorescence signal is demodulated at that samefrequency, yielding independent lock-in signals for the n NV resonancesfrom a single optical measurement. In this way, adding n channelsrequires no additional hardware or measurement time and involves minimaladditional processing overhead. Note that there is now additionaltechnical complexity in that it is important to choose now n modulationfrequencies that are far enough away from the other modulationfrequencies to avoid unintended cross-talk and inconveniently locatedintermodulation distortions, while making sure all modulationfrequencies are still in the range for optimal sensitivity.Alternatively, the modulation frequencies of some channels may be chosento be slightly outside the range for optimal sensitivity, therebytrading off better trade-off for reducing cross-talk effects between thechannels.

To implement this simultaneous vector magnetometry approach, one couldfirst perform an ODMR sweep to locate the positions of all 8 NVresonances, and then apply the n-channel frequency-lockingimplementation to lock to the upper and lower resonances of either 4 NVorientation classes, or 3 NV orientation classes. The benefits to one orthe other are similar to those described in the previous section; here,the 3-NV-class (6-channel) implementation is technically simpler whilethe 4-NV-class (8-channel) implementation may afford better sensitivity.Again, as in the previous section, one could implement the 3-NV-class(6-channel) implementation and real-time update the measured NVorientation classes during the course of the measurement to optimizesensitivity.

This 6- or 8-channel simultaneous frequency-locking technique hasseveral major advantages over the other techniques discussed thus far;the simultaneous measurements of all resonances allows for the full andaccurate reconstruction of the magnetic field vector, decoupled fromtemperature effects, and immune to technical noise as afforded by thebasic frequency locking technique. This technique overcomes thesystematic errors due to magnetic fields and temperatures changing ontimescales that are comparable to the sequential cycling rate, inherentto sequential measurements. As a magnetic vector sensor that is robustagainst noise due to movement and vibration in an ambient magneticfield, the NV-diamond based vector magnetic sensor is quite well-suitedfor application on mobile platforms which may move and rotate in thepresence of an ambient magnetic field. Furthermore, the simultaneousmeasurements provide an additional enhancement to the magneticsensitivity by as much as a factor of 2, by virtue of the fact thatmeasurement duty cycle of each individual NV orientation class increasesfrom 25% (4-NV) or 33% (3-NV) in the sequential vector measurement caseto 100% in the simultaneous case discussed here.

Simultaneous Vector Magnetometry Via Simultaneous 4-ChannelFrequency-Locking

Another approach to performing measurements on multiple NV orientationclasses that both overcomes the systematics of the sequentialmeasurement and reduces the technical complexity compared to that of thesimultaneous 6- or 8-channel implementation is to perform simultaneousfrequency-locking measurements on a subset of 4 of the possible 8 NVresonances. Under most magnetic field configurations, measurement of 4NV resonances is sufficient information to fully extract the fullmagnetic field vector and temperature. Mathematically, this approach canbe understood intuitively as measuring 4 knowns (i.e., NV resonancefrequencies) in order to extract 4 unknowns (temperature and threecomponents of the magnetic field vector).

For verification, we numerically solved the NV ensemble Hamiltonianunder varying conditions of magnetic field strength, magnetic fieldorientation, temperature, NV resonance linewidth (related to T₂* whichin turn affects the magnetic sensitivity), and how many and which NVresonances are used to reconstruct the magnetic field and temperature.From this numerical analysis, we confirmed that with appropriate choiceof 4 NV resonances, the full magnetic field vector and temperature canbe extracted with minimal reduction in sensitivity compared to using 8NV resonances in the vector reconstruction. As one example, at a givenmagnetic field orientation A, the four NV resonances corresponding tothe lowest, 2^(nd) lowest, 3^(rd) highest, and 4^(th) highest transitionfrequencies may yield the best magnetic sensitivity, while at adifferent magnetic field orientation B, the four NV resonancescorresponding to the lowest, 2^(nd) lowest, highest, and 2^(nd) highesttransition frequencies may yield the best magnetic sensitivity. Similarto the techniques described in the previous sections, a possibleimplementation of an NV magnetometer may perform real-time processing toreconstruct the measured magnetic field, evaluate what subset of four NVresonances will yield the most sensitive measurement of the currentorientation of the magnetic field, and update the device to performmeasurements using these optimized NV resonances.

This simultaneous 4-channel measurement technique has a clear advantageover the simultaneous 8-channel measurement technique presented in theprevious section in terms of reduced technical complexity. Specifically,in the simultaneous 4-channel technique, 4 simultaneous modulationfrequencies are employed, allowing for greater separation between themodulation frequencies in order to reduce cross-talk between thechannels, while at the same time allowing all four modulationfrequencies to be chosen from within a frequency range that optimizesthe magnetic sensitivity.

Flexibility in determining which NV resonances to employ in themeasurement is another advantage that eases the technical complexity ofthe NV magnetic sensor implementation compared to the simultaneous8-channel measurement approach. Specifically, while a particular subsetof NV resonances may provide the most sensitive measurement for a givenmagnetic field orientation, the NV resonances employed in themeasurement may be set by other constraints: the bandwidth of a MWcomponent used for a specific application may limit the accessible NVresonances to, e.g., those with the four highest transition frequencies.The simultaneous 4-channel measurement technique has the flexibility totrade off optimum sensitivity for other technical constraints and maythus be employed in situations and for applications not accessible tothe other techniques.

One specific technical consideration is that employing 4 NV resonancesthat correspond to 4 different NV orientation classes is directlycompatible with the sideband modulation technique described in thesection describing the basic frequency locking technique. That is,driving one resonance (either the upper or the lower) of each NV classallows one to perform sideband modulation to drive all hyperfinetransitions without concern for the depopulation or first order coherentdriving effects that emerge when both upper and lower resonances of asingle NV orientation class are driven. This sideband modulationtechnique can be applied to all four NV resonances (provided that theyeach correspond to a unique NV orientation class) to significantlyimprove the responsivity of the NV system and thus the magneticsensitivity of the device.

Note that in addition to the advantages listed previously in thissection, the simultaneous 4-channel frequency-locking technique sharesmany of the same benefits of the simultaneous 8-channelfrequency-locking technique. First, simultaneous vector measurementsusing multiple NV resonances improves the magnetic sensitivity comparedto sequential vector measurements by eliminating dead time onmeasurements of individual NV resonances, such that each addressed NVresonance has a measurement duty cycle of 100%. Second, simultaneousmeasurements avoid the systematic errors and 1/f noise that areintroduced when a sequence of non-coincident measurements are taken ondifferent NV resonances. As mentioned previously, the simultaneous4-channel frequency-locking technique yields a slightly degradedmagnetic sensitivity compared to the simultaneous 8-channelfrequency-locking measurement and may require slightly higher real-timeprocessing overhead. Note however, the relative advantages of eachtechnique may be balanced and tailored to specific applications byemploying anywhere from a minimum of 4 NV resonances simultaneously to amaximum of 8 NV resonances simultaneously.

Example Concepts for Magnetometer Systems Process Diagram of an ExampleNV-Diamond Sequential Vector Magnetometer System

FIG. 3 illustrates a process 300 for NV magnetometry using frequencylocking. Briefly, as shown in the flow path of process 300 in FIG. 3,the crystal host containing color centers (e.g., NV centers) are firstexposed to the magnetic field to be measured in step 301. This magneticfield can be an external field of unknown or known magnitude anddirection or a magnetic field internal to the crystal host. The step 301of process 300 includes other methods of introduction of the magneticfield to the sensing magnetometer under other instances when themagnetic field may not be explicitly or actively applied to a crystalhost containing color centers, for example, when the magnetic field isintrinsic to a material containing the crystal host. (From an NV'sperspective, there may be no perceptible difference between these twosituations.)

Suitable optical excitation and optical detection systems excite and/ordetect fluorescence emitted by the color centers. Appropriate frequencylocking systems use the emitted optical signal to locate the NVresonance frequency arising from driven state transitions and lock tothat frequency in a closed-loop manner as explained in detailpreviously. As a brief review, however, an ensemble of NV centers in thecrystal host can be excited by incident light, for example, a laserlight source at a wavelength of 532 nm. The photoluminescence emitted bythe crystal can be modulated by modulating the microwave drive of an NVresonance at a given modulation frequency. This photoluminescence signalcan be focused by a suitable optical system onto an optical detector,which transduces the photoluminescence into an electrical signal (e.g.,a photocurrent or voltage whose amplitude varies with the intensity ofthe photoluminescence) that can be demodulated at the modulationfrequency to produce an error signal. Using the error signal as input toa feedback controller that adjusts the microwave drive frequency tominimize the error signal allows the microwave drive frequency to belocked to a chosen NV resonance.

For optimal effectiveness of the frequency-locking technique, havingreasonable initial values for the NV resonance frequencies can beimportant. These initial values can be extracted in step 305: thecrystal host with the applied magnetic field is subjected to microwave(MW) radiation swept in a target range of frequencies that encompassesthe frequencies of NV resonances expected to be generated in the diamondhost. For example, the microwave source can be swept from about 2.65 GHzto about 3.1 GHz encompasses all NV resonance frequencies in the casewhere the applied magnetic field magnitude is less than or equal to 80Gauss. The step 305 can be an initialization step carried out once inthe beginning of process 300 and used to extract initial values for all8 NV resonances corresponding to all 4 NV orientation classes.

In step 306 of this process, one of the four possible crystal axes maybe chosen for the next set of simultaneous frequency-lockingmeasurements of upper and lower resonance. For certain implementationsof this technique, the period of time that the chosen crystal axes issampled may be changed or kept fixed.

Steps 308-309 and 312-313 are performed simultaneously and correspond tomeasurements on the lower and upper resonances, respectively.Specifically, an initial value for the lower resonance frequency isestimated in Step 308; this initial value may be the last measured valueof the lower resonance frequency of the chosen crystal axis or may beextrapolated from the last n measured values of the lower resonance ofthe chosen crystal axis or may be extracted from the most recent or lastn reconstructions of the total magnetic field vector, for example.Different algorithms for estimating an initial value for the lowerresonance may be appropriate for different situations depending onconsiderations of, e.g., processing speed and/or accuracy of the initialestimate. Step 312 is the analog of Step 308, but addresses the upperresonance of the crystal axis.

Recall from previous discussion that due to possible depopulation andfirst order coherent driving effects, it is typically preferable toavoid driving the same hyperfine transition in both upper and lowerresonances. Consequentially, for example, one may drive the |m_(s),m_(I)>=[0,−1>→|−1,−1> hyperfine transition of the lower resonance andthe |m_(s), m_(I)>=[0,0>→|+1,0> hyperfine transition of the upperresonance. One variation of Steps 308 and 312 may also involve changingwhich hyperfine transitions are driven from sample to sample. Doing somay provide some degree of randomization averaging that may bebeneficial for certain applications.

In step 309 of the process 300, the estimated value for the lowerresonance is input as an initial value into the first frequency-lockingchannel, which locks the MW drive frequency to the first transitionfrequency in a closed-loop manner such that the MW source is shifted infrequency to reflect any change in the frequency of the NV resonancecaused by the magnetic field. The frequency-locking channel thusprovides a direct measure of the NV resonance frequency independent ofphenomenological variations in, e.g., laser or mw power. Note that, dueto the locking, the process is also generally robust to the initialtransition frequency estimate being offset from the resonance byreasonably small values. Consequently, the amount of processing timeinvolved in extracting an accurate estimate for the initial transitionfrequency, the initial settling time of the frequency-locking channel,the processing power involved, and the overall achievable sampling speedof the magnetometer are all considerations that may be optimized fordifferent applications. Step 313 is the analog of Step 309, butaddresses the upper resonance of the crystal axis.

The process 300 includes a step 315 to extract temperature and magneticfield vector information from the frequencies of the upper and lowerresonances of color centers (e.g., NV centers) along the chosen crystalaxis. As a simple example, the magnitude of the sensed magnetic fieldalong the chosen crystal axis can be approximated by computing thedifference between the first and second lock-in frequencies obtainedsimultaneously from the two channels of the lock-in amplifier followingsteps 309 and 315 described above.

Recall that the crystal host in this example contains NV centers ofeight possible orientations, corresponding to four different crystalaxes. Measurements of the magnetic field vector information can becarried out along all four crystal axes by measuring from ensembles ofNV centers belonging to each orientation class. This is indicated bystep 317 in process 300 showing how Steps 306-315 may be repeatedcontinuously for real-time measurements of the full magnetic fieldvector information, decoupled from temperature as indicated. Note that,as discussed previously, there are several approaches to choosing thecrystal axis in step 306 for sequential measurement samples. Forexample, three crystal axes are sufficient to fully reconstruct themagnetic field vector in the Cartesian coordinate system, decoupled fromtemperature; consequently, one implementation of the process may cyclebetween only three crystal axes or may change which crystal axes areemployed to optimize the sensitivity of the measurement. Anotherpossible implementation for either the 3-crystal-axes or 4-crystal-axescases is for the order of which crystal axis to be measured from sampleto sample to be randomized in order to mitigate potential systematicerrors from the sequential measurement scheme. Additionally randomizingsampling period may further mitigate systematic errors. Differentimplementations of step 306 may be appropriate for differentapplications with different requirements and constraints.

Finally, upon performing measurements along at least three of the fourcrystal axes, the temperature and magnetic field magnitude and directioninformation measured from three or four crystal axes can be combined toreconstruct the full magnetic field vector, decoupled from temperature.This magnetic field vector can be converted from the intrinsictetrahedral coordinate frame of the NV-diamond system to the Cartesiancoordinate space as indicated in step 319 of process 300. The result isreal-time measurements of the full magnetic vector, decoupled fromtemperature effects (due to the simultaneous measurement of lower andupper resonances) and robust against phenomenological noise, e.g., dueto mw and laser intensity noise (afforded by the use of thefrequency-locking technique).

Process Diagram of an Example NV-Diamond Simultaneous VectorMagnetometer System

FIG. 23 illustrates a process 2300 for simultaneous NV magnetometryusing frequency-locking. Briefly, as shown in the flow path of process2300 in FIG. 23, the crystal host containing color centers (e.g., NVcenters) are first exposed to the magnetic field to be measured in step2301. This magnetic field can be an external field of unknown or knownmagnitude and direction or a magnetic field internal to the crystalhost. The step 2301 of process 2300 includes other methods ofintroduction of the magnetic field to the sensing magnetometer underother instances when the magnetic field may not be explicitly oractively applied to a crystal host containing color centers.

Suitable optical excitation and optical detection systems excite and/ordetect fluorescence emitted by the color centers. Appropriate frequencylocking systems use the emitted optical signal to locate the NVresonance frequency arising from driven state transitions and lock tothat frequency in a closed-loop manner as explained in detailpreviously. As a brief review, however, an ensemble of NV centers in thecrystal host can be excited by incident light, for example, a laserlight source at a wavelength of 532 nm. The photoluminescence emitted bythe crystal can be modulated by modulating the microwave drive of an NVresonance at a given modulation frequency. This photoluminescence signalcan be focused by a suitable optical system onto an optical detector,which transduces the photoluminescence into an electrical signal (e.g.,a photocurrent or voltage whose amplitude varies with the intensity ofthe photoluminescence) that can be demodulated at the modulationfrequency to produce an error signal. Using the error signal as input toa feedback controller that adjusts the microwave drive frequency tominimize the error signal allows the microwave drive frequency to belocked to a chosen NV resonance.

For optimal effectiveness of the frequency-locking technique, havingreasonable initial values for the NV resonance frequencies can beimportant. These initial values can be extracted in step 2305: thecrystal host with the applied magnetic field is subjected to microwave(MW) radiation swept in a target range of frequencies that encompassesthe frequencies of NV resonances expected to be generated in the diamondhost. For example, the microwave source can be swept from about 2.65 GHzto about 3.1 GHz encompasses all NV resonance frequencies in the casewhere the applied magnetic field magnitude is less than or equal to 80Gauss. The step 2305 can be an initialization step carried out once inthe beginning of process 2300 and used to extract initial values for all8 NV resonances corresponding to all 4 NV orientation classes.

In step 2306 of this process, some subset of the possible resonances maybe chosen for performing continuous simultaneous frequency-lockingmeasurements. In the example case of NV centers in diamond, there are 8possible resonances, though as discussed previously simultaneousmeasurements of 4 NV resonances can be sufficient to extract the fullmagnetic field vector, decoupled from temperature. Consequently, atypical number of resonances to choose in step 2306 to simultaneouslymeasure may be between 4 and 8, inclusive.

Steps 2308-2309 and 2312-2313 are performed simultaneously, where steps2308 and 2309 correspond to measurements on the first chosen resonanceand steps 2312 and 2313 stand in for parallel measurements along theremaining n−1 chosen resonances. In some instances of the process 2300,an initial value for the first chosen resonance frequency may beestimated in Step 2308; this initial value may be extracted from theinitialization step 2306 or may be extracted from the most recent orlast n reconstructions of the total magnetic field vector, for example.Different algorithms for estimating an initial value for the lowerresonance may be appropriate for different situations depending onconsiderations of, e.g., processing speed and/or accuracy of the initialestimate. Step 2312 is the analog of Step 2308, but addresses theremaining n−1 resonances involved in the measurement.

Note that during the typical course of operation of the magnetometer, anew estimation of an initial value for a given resonance frequencycorresponding to steps 2309 and 2313 may not be necessary as the channelwill already be frequency-locked to the NV resonance. However, asdiscussed previously, there may be occasional situations (e.g., afterinitialization or to optimize sensitivity) that which specific NVresonances are involved in the measurement or even the number of NVresonances involved in the measurement may be changed. Following theseNV resonance updates, it may be suitable to extract an estimatedtransition frequency to use as an initial input value into thefrequency-locking channel.

Recall from previous discussion that due to possible depopulation andfirst order coherent driving effects, it is typically preferable toavoid driving the same hyperfine transition in both upper and lowerresonances. Consequentially, for example, one may drive the |m_(s),m_(I)>=[0,−1>→|−1,−1> hyperfine transition of the lower resonance andthe |m_(s), m_(I)>=[0,0>→|+1,0> hyperfine transition of the upperresonance. Alternatively, one may drive only one resonance correspondingto each crystal axis.

In step 2309 of the process 2300, if desired, the estimated value forthe 1st resonance can be input as an initial value into the firstfrequency-locking channel, which locks the MW drive frequency to thefirst transition frequency in a closed-loop manner such that the MWsource is shifted in frequency to reflect any change in the frequency ofthe NV resonance caused by the magnetic field. The frequency-lockingchannel thus provides a direct measure of the NV resonance frequencyindependent of phenomenological variations in, e.g., laser or mw power.Note that, due to the locking, the process is also generally robust tothe initial transition frequency estimate being offset from theresonance by reasonably small values. Consequently, the amount ofprocessing time involved in extracting an accurate estimate for theinitial transition frequency, the initial settling time of thefrequency-locking channel, the processing power involved, and theoverall achievable sampling speed of the magnetometer are allconsiderations that may be optimized for different applications. Step2313 is the analog of Step 2309, but addresses the remaining n−1 chosenresonances.

Once simultaneous measurements have been performed on a sufficientnumber of NV resonances, the measured frequencies can be employed toreconstruct the full magnetic field vector, indicated by step 2319 ofprocess 2300. These real-time measurements of the full magnetic vectorare decoupled from temperature effects (due to the simultaneousmeasurement of lower and upper resonances) and robust againstphenomenological noise, e.g., due to mw and laser intensity noise(afforded by the use of the frequency-locking technique).

On occasion, for some implementations of process 2300, the extractedreal-time magnetic field measurement may be used to improve themagnetometer operation. For example, as discussed previously, one maywish to change which resonances are employed in the measurement in orderto, e.g., maintain optimized magnetic sensitivity. In another example,one may which to change the number of resonances are employed in themeasurement in order to, e.g., additionally overdetermine the system forthe purpose of resolving ambiguities that occur in the presence ofcertain magnetic field configurations. This is indicated by the arrowstarting at step 2319 and ending at step 2306. Note that these optional,occasional updates of the measured resonances take very little overheadin terms of measurement time and may substantially improve thetime-averaged magnetic sensitivity and robustness of a magnetometer,depending on the application.

Block Diagram of an Example NV-Diamond Magnetometer System

FIG. 4A shows an example frequency-locking magnetometer 400, in the formof a block diagram. The example magnetometer in FIG. 4A includes threemodules or packages: a physics package 432, an electronics package 434,and a data processing package 436. The physics package 432 includes acrystal (e.g., diamond) host 402 containing NV centers or other colorcenters, a microwave delivery mechanism 406 of some geometry (e.g., anantenna or resonant cavity) to drive the diamond host 402 with microwaveexcitation, and optical signal collection elements 404 with anassociated optical detector 412 (for example, one or more siliconphotodetectors, possibly referenced against the optical excitationintensity produced by the Quantum State Control (laser) unit 422discussed later, in a noise-balancing modality) to collect and measurethe emitted optical signal and produce an optical readout 414. In someembodiments of the magnetometer 400, the physics package 432 can alsoinclude an axis selection unit 442, which may, for example, include anapplied magnetic field source 436 that splits the NV resonances, makingthem non-degenerate and able to be individually addressed via resonantmicrowave irradiation. This applied magnetic field source 436 mayproduce static or dynamically adjustable magnetic fields using, forexample, coils or permanent magnets, in some embodiments of themagnetometer 400. Alternative implementations of the axis selection unit442 not related to magnetic field production are also possible,depending on application.

The electronics package 434 of the magnetometer 400 can include anamplifier 416 that receives the optical read out 414 from the opticaldetector 412. The amplified optical signal can be input into an analogto digital convertor (ADC) 438, which can in turn be connected to aQuantum State Control Processor (QSCP) 418, also part of the electronicspackage. The QSCP 418 can receive digitized signals from the ADC 438 andprocess the signals. In some instances, the QSCP 418 can function as orbe a lock-in amplifier. The QSCP 418 may also send outputs to the axisselection unit 442 which can be a part of the physics package 432, insome instances.

The electronics package 434 can include a first Quantum State ControlRadio Frequency unit (QSC-RF) 420 that is connected to the QSCP 418 andis configured to control RF signal generation and delivery. The QSC-RFunit 420 can be configured to receive signals from and transmit signalsto the QSCP 418. The QSC-RF unit 420 can be connected to and sendsignals to a second amplifier 424 that in turn delivers an amplifiedmicrowave signal 410 to drive the diamond host 402 via the microwavedelivery unit 406. The electronics package 434 can also include a secondQuantum State (laser) unit 422 that is connected to and receives signalsfrom the QSCP 418 and the first QSC-RF 420. The second QSC-Laser unit422 can be configured to control the laser light that illuminates thecrystal host 402. The QSC-RF 420 and QSC-Laser 422 units, includingexample implementations, are discussed in more depth later.

The example frequency lock-in magnetometer illustrated in FIG. 4A canalso include a data processing component 436 that includes a raw outputunit 426 that may receive output signals such as the raw output from theADC 438 of the electronics package 434, the lock-in signal from the QSCP418 of the electronics package 434, and/or the MW drive frequency orfrequencies from the QSC-RF 420. This unit 426 can be connected to andsend signals to a vector reconstruction unit 430 that controls vectorreconstruction (for example, as described in step 319 of the process 300illustrated in FIG. 3) of the magnetic field extracted from measurements(for example, according to steps 301 to 315) along the various crystalaxes (for example, following steps 315 and 317). That is, the vectorreconstruction unit 430 receives signals that allow for estimations ofthe magnetic field magnitude, magnetic field direction, and temperaturecorresponding to the different crystal axes, for example, along thetetrahedral coordinate system of the NV center and reconstructs themagnetic field vector in the Cartesian coordinate system.

The data processing component 436 of the magnetometer 400 can alsoinclude a data analysis and output unit 437 that analyzes and generatesan output that includes the magnitude and direction of a sensed magneticfield.

One example implementation of the process for measuring a magnetic fieldB is as follows: a signal generator denoted by a quantum state control(RF signal) unit 420 outputs a microwave signal with center frequencyf_(LO), modulated at frequency f_(ref) with depth f_(dev) to produce atime-dependent frequency given by:

f _(mod)(t)=f _(LO) +f _(dev) cos(2πf _(ref) t)  (16)

The signal is then sent via an amplifier 424 to an antenna 406 whichdrives the diamond sample 402 with the modulated microwave field. Thefluorescence signal from the NV ensemble is collected through signalcollection unit 404 and detected with a photodetector 412. The resultingsignal 414 converted to a digital signal through the analog-to-digitalconverter (ADC) 438 and subsequently demodulated by a lock-in amplifier416. The phase of the lock-in amplifier 416 is set such that thein-phase component of the lock-in signal is positive (negative) iff_(LO) is slightly above (below) the NV transition frequency, whereasthe lock-in signal is zero when f_(LO) is directly on resonance with theNV transition frequency. A feedback compensator within the quantum statecontrol processor 418, comprising a discrete integrator controller (notshown), produces an error signal 440 (indicated by an arrow from theQSCP 418 to the QSC-RF 420) from the lock-in signal. The error signal440 in turn provides feedback to adjust the microwave source frequencyf_(LO), and lock it to the center of the NV resonance. Note that here wehave described an example implementation; there are a variety ofimplementations for each unit in the block diagram, each can be adaptedfor different applications with different requirements and constraints.

When appropriate during the process, the NV diamond host is opticallyexcited, for example, by laser light from another quantum state control(QSC-laser) unit 422. The QSC-laser unit 422 can be used to control thelaser light delivery and can in turn receive feedback or other inputfrom the quantum state RF control unit 420 and/or the quantum statecontrol processor 418 mediating the error signal among other sources ofinput.

In an n-frequency-channel implementation of this technique, each NVresonance can be modulated at a different frequency f_(ref,n) to allowfor demodulation of all channels simultaneously. In this way, individuallock-in signals and subsequent error signals are extracted for eachfrequency channel concurrently.

The dynamics of the feedback loop can be analyzed by forming a linearapproximation to the system with an integrator controller to increasethe open-loop DC gain to minimize steady-state error. Examining the openloop transfer function,

${{L(z)} = {\frac{2{CV}_{0}}{\sigma^{2}}f_{dev}K_{1}{\frac{\alpha \; z}{z + \alpha - 1} \cdot \frac{z}{z - 1}}}},$

shows that the open loop gain depends on the following experimentalparameters: contrast (C), fluorescence amplitude (V₀), integrator gain(K_(I)), and resonance linewidth (σ). In a deployable system whereperiodic calibration is impractical, laser and microwave fluctuationsmay affect the accuracy of the sensor. The addition of a feedback loopallows locking into the frequency of the resonances. This closed looptransfer function is given by

${{T(z)} = \frac{L(z)}{1 + {L(z)}}},$

removing the dependence on these scale factors.

The example magnetometer system described above and shown in FIG. 4A canbe used to carryout measurements along all NV axes using the axisselection mechanism 442, which may, for example, include an appliedmagnetic field source 436 that splits the NV resonances, making themnon-degenerate and able to be individually addressed via resonantmicrowave irradiation. This applied magnetic field source 436 mayproduce static or dynamically adjustable magnetic fields using, forexample, coils or permanent magnets, in some embodiments of themagnetometer 400. Alternative implementations of the axis selection unit442 not related to magnetic field production are also possible,depending on application. This axis selection mechanism 422 may bedynamically adjusted by the quantum state processor 418.

Compact NV-Diamond Magnetometer

FIG. 5 shows an example compact magnetometer 500 with some of theindividual components outlined in FIG. 4 assembled into a compactmodule. In this example, the physics module 532 includes, among othercomponents, an NV-diamond sensor 502, an RF signal delivery system 506,a bias magnetic field delivery system 536, a control laser deliverysystem 538 from a control laser source 508, optical collection optics514, and a read-out diode detector 512. The magnetometer 500 may alsoinclude an electronics module 534 to implement frequency-locking. TheNV-diamond sensor 502 can be fabricated precisely and cut with knowncrystal axes. The RF signal delivery system 506 can be controlled by aquantum state control unit (not shown in FIG. 5) and coupled to aquantum state control processor unit (also not shown in FIG. 5) todeliver the appropriate microwave radiation signal to drive theNV-diamond sensor 502. The bias magnetic field delivery system 536 canbe configured to deliver the magnetic field. The control laser deliverysystem 538 can be controlled by a quantum state control unit (not shownin FIG. 5) to deliver, e.g., laser light from a control laser source 508onto the NV diamond sensor 502. The control laser source 508 can be abroad band, high-bandwidth source with low relative intensity noise toensure optimal excitation and reduced or minimized noise forfrequency-locking. The compact magnetometer 500 shown in FIG. 5 can alsobe configured to be a hand-held magnetometer measuring no more than sixinches of length, as indicated in FIG. 5. It could also be packaged in alarger form, e.g., suitable for a bench top or mounting on a breadboard.

Note that this example compact magnetometer 500 illustrates some ways inwhich the NV system is particularly well-suited as a compactmagnetometer. For example, the solid-state diamond crystal host allowsfor a very high density of NV centers to be incorporated into a verysmall NV-diamond sensor 502, which likewise relaxes size, uniformity,and complexity constraints many of the other components, such as themagnetic bias field delivery 536 and RF signal delivery 506 for example.Furthermore, control of the NV system requires fairly low overhead,indicated by the relatively relaxed specifications for, e.g., thecontrol laser 508.

Experimental Demonstrations of Magnetometer Systems Simultaneous2-Channel Frequency-Locking

In an example experiment, a dual-channel, frequency locking NVmagnetometer was implemented by employing a field-programmable gatearray (FPGA) (commercially available) and a high-speeddigital-to-analog-converter to digitally synthesize two carrier signalswith frequencies that could be independently tuned and modulated. Thegenerated signal was amplified with an amplifier (AMF-4B-02000400-20-33PLPN, Miteq) and sent to a loop antenna to produce a microwave field atthe diamond sample, which contained a large ensemble of NV centers(˜1×10¹³). The diamond sample was optically excited with a 532-nm laser(Verdi V8, Coherent) in a light-trapping diamond waveguide geometry forincreased optical excitation efficiency. The resulting fluorescence wascollected with a balanced photodetector (New Focus) where a pick-offfrom the laser was directed into the balancing port to remove commonmode laser noise. The lock-in amplifiers for demodulating the balancedphotodetector signal and feedback compensators for locking to the NVresonances were also programmed into the FPGA, which continuously putout the two locked frequencies to a computer.

Note that the effectiveness of the photodetector in balancing out laserintensity noise can have a significant effect on the technical noisefloor of the NV magnetometer system and consequently on the magneticsensitivity of the measurements. For example, recall that in FIG. 10A,in addition to the expected 1/f noise<2 kHz, there is noise above theshot noise limit at modulation frequencies>6 kHz. Poor photodetectorbalancing may be the source of either or both of these noise features,which have deleterious effects, such as limiting the range of modulationfrequencies that provide optimal sensitivity in lock-in basedtechniques.

Another possible source of noise in the photodetector in particular isnoise from a power source being coupled into the photodetector biasvoltage. FIGS. 9A and 9B show the noise spectral density plot of dataobtained during magnetic field measurements superimposed by expectedestimation of noise floor, from a single channel of the amplifier undertwo conditions. FIG. 9A shows noise spectral density when the apparatusof the magnetometer was powered by an external power supply, and FIG. 9Bshows the noise spectral density when the apparatus was powered by abattery. As shown the measurements were above the shot-noise limit bythe expected amount given 1/f noise in the laser over the frequencyrange shown, for both power sources. Specifically, the sensitivity at apower of 1.8 W was measured to be 172 pT/Hz^(1/2). These measurementsillustrate that proper choice of an external power supply does notcontribute to the technical noise as compared a batter, canonically themost quiet power source. However, note that the measurement noise wasabove the shot noise limit by a factor of 1.6 indicating that in thisexample system, technical noise can be improved.

Permanent magnets arranged in a Halbach array configuration produced auniform magnetic bias field B₀≈7.8 mT over the diamond such that each ofthe four possible NV orientation classes in the diamond crystalexperienced a different magnetic field projection. The dual-channelfrequency-locking NV magnetometer simultaneously locked to both them_(s)=0→±1 transitions of a single NV orientation in order to decouplethe effects of temperature and magnetic field and thus extracted themagnetic field projection along the NV symmetry axis.

Applying simultaneous microwave driving at both transition frequenciesof an NV orientation class can produce a depopulation effect, whichchanges the shape of the ODMR spectra at one resonance due to the driveapplied at the corresponding resonance. However, due to hyperfineinteractions between the NV electronic spin and the nuclear spin I=1 ofthe more naturally abundant ¹⁴N isotope, each NV m_(s)=0→±1 transitionwas composed of three transitions separated by ˜2 MHz (see example ODMRspectrum in FIG. 2B). Consequently, one frequency channel was set to the|0, −1>→|−1, −> transition with the other channel set to |m_(s),m_(I)>=|0, 0>→|+1, 0> transition to avoid this depopulation effect.

FIG. 7A demonstrates the robustness of the frequency-locking NVmagnetometer against changes in phenomenological variables, such asoptical pump and microwave drive power, in an example implementation.The m_(s)=0→±1 transitions of the NV orientation class that is mostaligned to the applied magnetic field were frequency locked with, usingmodulation frequencies f_(ref,1)=3 kHz and f_(ref,2)=4 kHz andmodulation depths f_(dev,1,2)=500 kHz. Upon varying the optical pumppower, drastic differences in the contrast of the ODMR resonance wereobserved (upper inset), which corresponded to different slopes in thelock-in signal (lower inset). Under these circumstances, previouslock-in-based implementations measured changes in the magnetic fieldthat are off from the actual values, with the discrepancy beingproportional to the difference between the current lock-in slope and thelast calibration of the lock-in slope. Using the inventive frequencylocking technique applied to two or more NV resonances, simultaneously,even while large variations in the optical pump power can result invarying transient responses to a step input, the measured steady-statemagnetic field remained consistent.

Employing a permanent magnet to vary the magnetic field experienced bythe frequency-locking NV magnetometer, a dynamic range˜4 mT could bedemonstrated as shown in FIG. 7B. The dynamic range of this techniquecould reach to ˜10 mT limited only by spin state mixing destroyingfluorescence signal contrast. In contrast, in an open-loop lock-in basedimplementation, the dynamic range was limited to the linear region ofthe ODMR derivative signal,

${\approx \frac{h\; \sigma}{4g_{e}\mu_{B}}},$

where σ is the linewidth of the NV resonance. For a typical σ≈1 MHz, thedynamic range was 10 μT, approximately one order of magnitude less thanthe range demonstrated here and three orders of magnitude less than theexpected limit. Furthermore, the frequency-locking technique allows forthe magnetic sensitivity to be maintained over the full dynamic range ofthe NV center. FIG. 7C shows magnetic sensitivity illustrated in thenoise spectrum of an example measurement using the magnetometerdescribed above.

While the dynamic range of the current dual channel frequency locking NVmagnetometer may be limited the 2.4 GHz bandwidth of the MW amplifier,this can be easily addressed by employing additional or alternativeelectronics. Simultaneous detection of fields along all four possiblediamond tetrahedral directions and performing real-time analysis to takeadvantage of the redundancy of the overdetermined system can helpovercome the ambiguity inherent in measurements where individual NVresonances cross and/or are degenerate at crossover points. This can beachieved by implementing an 8-channel magnetometer capable of robustlytracking the magnetic field through ambiguous crossover points.

In this experimental demonstration, without sacrificing sensitivity ormeasurement speed, measurements achieved a dynamic range of about 4 mT,corresponding to a factor>10 improvement over some previousimplementations of lock-in-based NV magnetometers. This versatility isextremely useful for sensing previously unknown magnetic fields wherevariations over a wide range are expected.

Sequential Vector Magnetometry Via Simultaneous 2-ChannelFrequency-Locking

In one implementation of the magnetometer detailed above, a Halbacharray of permanent magnets applied a uniform magnetic field over an NVensemble within a diamond sample causing Zeeman splitting in them_(s)=0→±1 transitions of the four NV orientation classes and producingeight distinct NV resonances, each with three hyperfine transitions. Theresults are shown in FIG. 6A. The microwave frequency was swept from2.65 GHz to 3.1 GHz.

The magnetometer was simultaneously frequency-locked to the [m_(s),m_(I))=[0, −1)→[−1, −1) and [0,0)→[+1,0) hyperfine transitions of asingle NV orientation class (corresponding to the thin solid and dashedlines, respectively, in FIG. 6A) and sequentially iterated through eachNV orientation, with a 0.1-second dwell time per resonance pair. Thevertical lines indicate the frequencies that were locked to.

A set of three-axis coils applied additional 10 μT magnetic fields alongthree orthogonal directions. Specifically, a 0.2 G field was applied inthe x direction for 10 seconds, then the y direction, and then the zdirection. FIG. 6B shows the resulting NV resonance frequency shifts ofeach of the eight resonances in FIG. 6A, as a function of time, detectedusing this method of locking to four pairs of NV transitions in rapidsequence. Using the NV Hamiltonian and the known, rigid tetrahedralgeometry of diamond crystal makes it possible to reconstruct themagnetic field vector by performing non-linear optimization of theover-constrained system, which resulted from having measurements of themagnetic field projections along four directions rather than three.

FIG. 6C shows magnetic field components reconstructed from the frequencydata in FIG. 6B, according to the step 319 in the process 300 in FIG. 3.The illustrations on the left show the tetrahedral frame of reference ofthe NV axes and the Cartesian frame of reference in which thedirectionality of the magnetic field is finally determined.

The reconstructed magnetic field vector presented in FIG. 6C was inexcellent agreement with the expected magnetic field produced by thecalibrated three-axis coils. Notably, the tetrahedral co-ordinate systemof the NV center served as an optimal measurement system with aredundant axis, thereby providing 2/√3 improvement in sensitivity over ameasurement made in the Cartesian system. Further, failure along any oneaxis could be tolerated with loss in sensitivity of at most a factor of2.

Simultaneous Vector Magnetometry Via Simultaneous 8-ChannelFrequency-Locking

In one implementation of an NV vector magnetometer, detailed above, aHalbach array of permanent magnets applied a uniform magnetic field overan NV ensemble within a diamond sample causing Zeeman splitting in them_(s)=0→±1 transitions of the four NV orientation classes and producingeight distinct NV resonances, each with three hyperfine transitions. Toperform an initial demonstration of the simultaneous vector magneticsensing capability, we mounted the example NV magnetometerimplementation and a commercial fluxgate sensor in proximity to eachother in an unshielded lab environment.

Performing simultaneous 8-channel frequency-locking measurements allowedfor the extraction of all 8 NV resonance frequencies simultaneously. Inthis implementation, the 8 modulation frequencies were chosen fromwithin the range 2.5 kHz to 3.7 kHz. The magnetic field projection alongeach of the four crystal axes could be extracted by using the 8resonance frequencies to numerically solve the NV ensemble vectorHamiltonian for the full magnetic field vector, decoupled fromtemperature. In a simple example, however, the magnetic field projectionalong each crystal axis may approximated by dividing the differencebetween the upper and lower resonance frequency corresponding to eachorientation class by the fundamental-constant-dependent Zeeman splittingterm, as indicated previously in Equation (14).

Noise spectra could then be extracted from these measurements of themagnetic field along each of the crystal axes both as an indication ofthe magnetic sensitivity achieved by each NV orientation class and as anindication of potential crosstalk between the 8 frequency-lockingchannels. These noise spectra are shown in FIGS. 21A-21D for each of theNV orientation classes, labeled NV A, NV B, NV C, and NV D,respectively. Note that each noise spectrum has roll-off behavior forhigher measurement frequencies (>10 Hz in this example case),attributable to a combination of the lock-in parameters and thefrequency-locking feedback controller parameters. The time constant andsteepness of this roll-off can be easily adjusted to be suitable fordifferent applications by modifying the lock-in and frequency-lockingfeedback controller parameters, even dynamically during a measurement.

Note from the noise spectra of an example measurement from the exampleimplementation that in this case, while there are significant cross-talkeffects in the 1-10 kHz range and some additional cross-talk effects inthe 10 Hz-1 kHz range, there are no observable cross-talk effects in themeasurement bandwidth of the example simultaneous vector magnetometerimplementation. Furthermore, generally, the position of the cross-talkfeatures as well as the strength of their effects can be adjusted withcareful choice of the modulation frequencies, lock-in parameters, and/orfrequency-locking feedback controller parameters. Consequently,cross-talk effects can generally be minimized and/or made negligible byadjusting the modulation frequencies employed in the frequency-lockingchannels, the lock-in parameters, and the frequency-locking feedbackcontroller parameters as appropriate for specific applications.

Finally, we can compare the measured sensitivity extracted from thenoise floor of the measurement over frequencies<10 Hz against theexpected the expected shot-noise-limited sensitivity for each NVorientation class and find that they are in good agreement. Recall thatthis measurement represents an initial demonstration of a simultaneousvector magnetometer using the simultaneous 8-channel frequency lockingtechnique; consequently there is significant room for improving variousexperimental parameters to optimize sensitivity.

To demonstrate the simultaneous vector capability of the example NVmagnetometer implementation, we mounted it and a commercial fluxgatesensor in proximity to each other in an unshielded lab environment. Wethen moved large magnetic objects around far away from the sensors toensure that both devices experienced similar magnetic field variations.FIGS. 22A and 22B show agreement between the magnetic field projectionsmeasured by the NV apparatus and the commercial fluxgate sensor. Thisrepresents the first demonstration of simultaneous vector magnetometrywith NV centers in diamond, tied to fundamental constants with vectoraxes set by the inherent, stable diamond crystal lattice.

CONCLUSION

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of” “only one of” or“exactly one of.” “Consisting essentially of” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in Section 2111.03 of the United StatesPatent Office Manual of Patent Examining Procedure.

1. A method of measuring a magnetic field with a solid-state hostdisposed within the magnetic field, the solid-state host comprising afirst ensemble of color centers oriented along a first crystal axis ofthe solid-state host, the first ensemble of color centers exhibiting afirst resonance in a presence of the magnetic field, the methodcomprising: frequency-locking a first microwave signal to the firstresonance; and determining an amplitude and/or direction of the magneticfield based on a frequency of the first microwave signal.
 2. The methodof claim 1, wherein frequency-locking the first microwave signal to thefirst resonance comprises shifting the frequency of the first microwavesignal in response to a change in the magnetic field.
 3. The method ofclaim 1, wherein the first resonance represents a first energy-leveltransition.
 4. The method of claim 3, wherein the first energy-leveltransition is a first hyperfine transition.
 5. The method of claim 1,wherein determining the amplitude and/or direction of the magnetic fieldcomprises determining the amplitude of the magnetic field with a dynamicrange of about 360 μT to about 100,000 μT.
 6. The method of claim 1,wherein the first ensemble of color centers exhibits a second resonancein a presence of the magnetic field, and further comprising:frequency-locking a second microwave signal to the second resonance, andwherein determining the amplitude and/or direction of the magnetic fieldis based on a frequency of the second microwave signal.
 7. The method ofclaim 6, wherein the second resonance represents a second energy-leveltransition different than the first energy-level transition.
 8. Themethod of claim 6, wherein determining the amplitude and/or direction ofthe magnetic field comprises taking a difference between the frequencyof the first microwave signal and the frequency of the second microwavesignal.
 9. The method of claim 1, wherein the solid-state host comprisesa second ensemble of color centers oriented along a second crystal axisof the solid-state host and exhibiting a second resonance and a thirdensemble of color centers oriented along a third crystal axis of thesolid-state host and exhibiting a third resonance in the presence of themagnetic field, and further comprising: frequency-locking a secondmicrowave signal to the second resonance; and frequency-locking a thirdmicrowave signal to the third resonance.
 10. The method of claim 9,wherein determining the amplitude and/or direction of the magnetic fieldis further based on a frequency of the second microwave signal and afrequency of the third microwave signal.
 11. A system for measuring amagnetic field, the system comprising: a solid-state host comprising afirst ensemble of color centers oriented along a first crystal axis ofthe solid-state host, the first ensemble of color centers exhibiting afirst resonance in a presence of the magnetic field; a microwave signalgenerator, in electromagnetic communication with the solid-state host,to drive the first ensemble of color centers with a first microwavesignal; a photodetector, in optical communication with the solid-statehost, to detect a first fluorescence signal emitted by the firstensemble of color centers, the first fluorescence signal representingthe first resonance; circuitry, operably coupled to the microwave signalgenerator and the photodetector, to frequency-lock the first microwavesignal to the first resonance based on the first fluorescence signal;and a processor, operably coupled to the lock-in amplifier, todetermining an amplitude and/or direction of the magnetic field based ona frequency of the first microwave signal.
 12. The system of claim 11,wherein the circuitry is configured to shift the frequency of the firstmicrowave signal in response to a change in the magnetic field.
 13. Thesystem of claim 11, wherein the first resonance represents a firstenergy-level transition.
 14. The system of claim 13, wherein the firstenergy-level transition is a first hyperfine transition.
 15. The systemof claim 11, wherein the processor is configured to determine theamplitude and/or direction of the magnetic field with a dynamic range ofabout 360 μT to about 100,000 μT.
 16. The system of claim 11, whereinthe first ensemble of color centers exhibits a second resonance in apresence of the magnetic field, the circuitry is configured tofrequency-lock a second microwave signal to the second resonance, andthe processor is configured to determine the amplitude and/or directionof the magnetic field based on a frequency of the second microwavesignal.
 17. The system of claim 16, wherein the second resonancerepresents a second energy-level transition different than the firstenergy-level transition.
 18. The system of claim 16, wherein theprocessor is configured to determine the amplitude and/or direction ofthe magnetic field comprises by taking a difference between thefrequency of the first microwave signal and the frequency of the secondmicrowave signal.
 19. The system of claim 11, wherein the solid-statehost comprises a second ensemble of color centers oriented along asecond crystal axis of the solid-state host and exhibiting a secondresonance and a third ensemble of color centers oriented along a thirdcrystal axis of the solid-state host and exhibiting a third resonance inthe presence of the magnetic field and wherein the circuitry isconfigured to frequency-lock a second microwave signal to the secondresonance and to frequency-lock a third microwave signal to the thirdresonance.
 20. The system of claim 19, wherein the processor isconfigured to determine the amplitude and/or direction of the magneticfield based on a frequency of the second microwave signal and afrequency of the third microwave signal.